IIEXLIBRIS 


THE      SUN 


THE     SUN 


BY 


CHARLES   G.  ABBOT,  S.M. 

DIRECTOR  SMITHSONIAN   ASTROPHtSICAL   OBSERVATORY 


WITH   NUMEROUS   ILLUSTRATIONS 


NEW     YORK    AND    LONDON 
D  .     A  P  P  L  E  T  O  N     AND     COMPANY 

1911 


COPYRIGHT,  1911,  BY 
D.  APPLETON  AND  COMPANY 


Published  September,  1911 


Printed  in  the  United  States  of  America 


PEEPACE 


WITHIN  the  last  fifteen  years  we  have  seen  the  publi- 
cation of  Rowland's  great  table  of  solar  spectrum 
wave  lengths,  the  establishment  of  the  Yerkes,  Ko- 
daikanal,  Mount  Wilson  and  other  observatories 
largely  devoted  to  solar  researches,  the  photography 
of  the  spectrum  of  the  corona  and  of  the  chromosphere 
at  total  solar  eclipses,  Hale's  brilliant  discovery  of 
magnetic  fields  in  sun  spots,  the  determination  of  the 
rotation  periods  of  the  sun  at  different  levels,  as  well  as 
at  all  solar  latitudes,  Langley's  bolometric  investigation 
of  the  sun's  infra-red  spectrum,  and  the  recent  Smith- 
sonian determinations  of  the  absolute  intensity  of  the 
solar  radiation  outside  our  atmosphere.  The  great 
interest  in  such  researches  has  been  marked  by  the 
establishment  of  the  International  Solar  Union,  and  its 
enthusiastic  gatherings  of  the  foremost  investigators 
from  all  lands. 

The  tune  seems  ripe  for  collecting  the  splendid  array 
of  new  solar  knowledge  which  such  unprecedented 
activity  has  produced,  and  for  discussing  the  probable 
nature  of  the  sun  in  the  light  gained. 

In  the  following  pages  the  professional  astronomer 
will  find  hitherto  unpublished  results  of  researches, 
and  new  explanatory  hypotheses,  illustrated  by  many 
new  text  figures  and  engravings. 

v 

223896 


PREFACE 

Chapter  II  has  been  devoted  to  a  description  of  the 
methods  and  principles  employed  in  modern  solar 
research,  and  Chapters  VII  to  X  on  the  relations  of  the 
sun  to  life  upon  the  earth,  and  to  the  starry  universe  in 
general.  Thus  the  book,  while  primarily  devoted  to 
the  sun,  may,  I  hope,  serve  as  an  introduction  to  the 
study  of  astrophysics  for  school  and  college  use,  as 
well  as  for  the  general  reader. 

In  Chapters  VI  to  IX  are  given  many  facts  likely 
to  prove  of  interest  to  the  meteorologist,  geologist, 
botanist  and  engineer. 

Professor  Young's  "The  Sun"  is  now  out  of  print, 
and  as  it  is  hoped  that  the  present  work  may  to  some 
extent  take  its  place,  I  have  been  permitted  to  use 
some  of  his  illustrations,  notably  in  Chapter  IV,  and 
to  make  several  quotations  from  his  text,  the  longest 
in  Chapters  IV  anil  VI.  I  desire  also  to  acknowledge 
my  obligations  to  many  who  have  given  suggestions, 
information  and  illustrations.  Especially  I  offer  thanks 
to  F.  E.  Fowle,  S.  A.  Mitchell,  W.  W.  Campbell,  G.  E. 
Hale  and  the  staff  of  the  Mount  Wilson  Solar  Observa- 
tory, E.  B.  Frost,  H.  M.  Chase,  A.  G.  Eneas,  Henry 
Holt  and  Company,  the  Superintendent  of  the  United 
States  Naval  Observatory,  M.  J.  Moore  of  the  United 
States  Patent  Office,  and  Messrs.  Briggs  and  Shantz  of 
the  United  States  Department  of  Agriculture. 


C.  G.  ABBOT. 


WASHINGTON, 
July  10/1911. 


CONTENTS 


CHAPTER  I 

PAGES 

THE  SOLAR  SYSTEM.    THE  SUN'S  DISTANCE.    ITS  DIMENSIONS  1-30 

CHAPTER  II 

THE  INSTRUMENTS  AND  METHODS  USED  IN  SOLAR  INVESTI- 
GATION         .  31-84 

The  Telescope. — The  Coelostat. — The  Spectrum  and 
What  It  Indicates.  —  Spectroscopes.  —  The  Spectro- 
heliograph. — The  Heliomicrometer. — The  Comparator. 
— Nature  of  Radiation  — Laws  of  Radiation. — Spectra 
of  Different  Sources. — Pyrheliometry.-^Bolometry. 

CHAPTER  III 

THE  PHOTOSPHERE 85-127 

Telescopic  View. — The  Photospheric  Spectrum. — Row- 
land's Spectrum  Tables. — Chemical  Elements  Found 
and  not  Found. — Corrections  to  Rowland's  Wave- 
lengths .  — Levels .  — Pressures.  — Convection  Currents .  — 
Limb  Spectra. — The  Variation  of  the  Sun's  Brightness. — 
Solar  Temperatures. — Spectro-heliography. — The  Solar 
Rotation. 

CHAPTER  IV 

ECLIPSES  AND  THE  OUTER  SOLAR  ENVELOPES        .       .     128-182 

The  Saros. — Eclipse  Expeditions. — The  Corona. — The 

Chromosphere. — Jansen's  and  Lockyer's  Discovery. — 

The  Spectra  of  tte  Chromosphere  and  Prominences. — 

vii 


CONTENTS 

PAGES 

Prominences  and  the  Spectroheliograph. — Recent  Flash 
Spectrum  Observations. — The  Heights  of  Different 
Metals  in  the  Chromosphere. — Mitchell's  Observations 
of  1905. — Campbell's  Observations. — Chromospheric 
Spectra  in  Full  Daylight. 

CHAPTER  V 

SUN-SPOTS,  FACULAE  AND  GRANULATION  .  .  .  183-214 
Sun-spot  Periodicity  — Drift — The  Distribution  of  Sun- 
spots. — Their  Formation  and  Life  History. — The  Sun- 
spot  Level. — Langley's  Typical  Sun-spot. — Faculae. — 
Granulation. — Sun-spot  Spectra. — Sun-spots  and  Mag- 
netism.— Radial  Motion  in  Spot  Penumbras. 

CHAPTER  VI 

WHAT  is  THE  SUN? 215-279 

Young's  Views. — Halm's  Views. — Schmidt's  Hypothesis. 
— Julius'  Views*. — The  Author's  Views. 

CHAPTER  VII 

THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT  .  .  280-330 
Causes  of  Low  Temperatures  at  High  Altitudes. — Meas- 
urement of  the  Intensity  of  Sun  Rays. — Dependence  of 
Solar  Radiation  on  Air  Mass  — The  Transmission  o'f  the 
Atmosphere. — The  "Solar  Constant  of  Radiation." — 
The  Light  of  the  Sky.— The  Dependence  of  the  Earth's 
Temperature  on  Radiation. — The  Fluctuation  of  Solar 
Emission. — Geological  Temperatures. 

CHAPTER  VIII 

THE  SUN'S  INFLUENCE  ON  PLANT  LIFE    ....     331-361 
Plant  Requirements. — The  Assimilation  of  Carbon  by 
Autotrophic  Plants. — Etiolation. — Plant  Geography. — 
Light  Requirements  of  Plants. — Heliotropism. — Plants 
as  Energy  Accumulators. 

viii 


CONTENTS 
CHAPTER  IX 

PAGES 

UTILIZING  SOLAR  ENERGY 362-390 

Experiments  with  Burning  Mirrors. — The  "Hot-box" 
Principle. — Mouchot,  Pifre  and  Ericsson. — Eneas'  Solar 
Engines. — Properties  of  Glass. — Solar  Heaters  and  Res- 
ervoirs.— Low  Temperature  Solar  Engines. — Solar 
Cooking  Appliances. — Solar  Metallurgy. — Resume*.— 
Quantity  of  Solar  Energy  Available. — Thermo-dynamic 
Efficiency. — Reflecting  Powers  of  Mirror  Surfaces. 

CHAPTER  X 

THE  SUN  AMONG  THE  STARS  ' 391-434 

Stellar  Distances. — Magnitudes. — The  Sun's  Magnitude 
and  Light  Emission. — The  Solar  Motion. — Star  Groups. 
— Double  Stars. — Stellar  Masses  and  Distances. — Mira 
Ceti  and  the  Sun. — Stellar  Spectra. — The  Classification 
of  Stellar  Spectra. — Radiation  Distribution. — Evolution 
of  the  Solar  System. — Stellar  Evolution 

CONCLUSION 435-437 

INDEX  439-448 


LIST  OF  PLATES  AND   ILLUSTRATIONS 
IN  TEXT 


LIST    OF    PLATES  FACING 

PAGE 

Langley's  typical  sun-spot Frontispiece 

PLATE  I. — Smithsonian    observing    shelter    and    coelostat 

(Mount  Wilson) 36 

PLATE  II.— The  Zeeman  effect  (King)      .       .       .       .       .44 

PLATE  III. — Solar  photograph  (Ellerman)        ....  86 
PLATE  IV. — Fig.  1.  'Spectrum  of  procyon  (Adams);  Fig.  2. 

Spectrum  of  east  and  west  limbs  of  the  sun  (St.  John)  .  88 
PLATE  V. — Calcium  spectroheliogram,  H3  (Ellerman)   .        .114 

PLATE  VI. — Hydrogen  spectroheliogram,  Ha  (Ellerman)      .  116 

PLATE  VII. — Calcium  spectroheliogram,  Ht  (Ellerman)        .  118 

PLATE  VIII. — Calcium  spectroheliogram,  Ha  (Ellerman)      .  120 

PLATE  IX. — Calcium  spectroheliogram,  H3  (Ellerman)        .  122 

PLATE  X. — Hydrogen  spectroheliogram,  Hy  (Ellerman)       .  124 

PLATE  XI. — Hydrogen  spectroheliogram,  Ha  (Ellerman)     .  126 
PLATE  XII.— Solar  corona,  1900,  May  28              .       .       .132 

PLATE  XIII.— Solar  corona,  1905,  August  30         ...  134 
PLATE  XIV. — Spectroheliograms  of  solar  prominences  (Slo- 

cum) 166 

PLATE  XV.— Flash  spectra,  1905,  August  30  (S.  A.  Mitchell)  174 

PLATE  XVI. — Photographs  of  a  portion  of  the  sun  (Jansen)  202 
PLATE  XVII. — Spectra  of  photosphere  and  sun-spot  (Mount 

Wilson  Solar  Observatory) 210 

PLATE  XVIII. — Effect  of  pressure  on  gaseous  spectra  (Gale)  248 

PLATE  XIX.— The  Pleiades  (Ritchey) 398 

PLATE  XX  A.— Stellar  spectra  (Campbell)      .       .       .       .406 

PLATE  XX  B.—  Stellar  spectra  (Campbell)      ....  408 

PLATE  XXL— Stellar  spectra  (Hale  and  Ellerman)      .       .  410 

xi 


PLATES  AND  ILLUSTRATIONS   IN  TEXT 

FACING 
PAGE 

PLATE  XXII.— The  great  nebula  in  Orion  (Ritchey)  .  .  •  418 

PLATE  XXIII.— Nebula  N.  G.  C.  6992  Cygni  (Ritchey)  .  420 

PLATE  XXIV. — The  great  nebula  in  Andromeda  (Ritchey)  422 
PLATE  XXV. — Spiral  Nebula  M.  51  Canum  Venaticorum 

(Ritchey) 424 

PLATE  XXVI.— Fig.  1.  Nebula  H.  V.  24  (Ritchey);  Fig.  2. 

Ring  nebula  in  Lyra  (Ritchey) 426 


LIST  OF  ILLUSTRATIONS  IN  TEXT 


PAGE 


1. — Relative  distances  of  planets 5 

2. — Geometrical  parallax  method 10 

3.— Velocity  of  light  (Fizeau) 19 

4.— Velocity  of  light  (Foucault) 20 

5. — Determination  of  constant  of  gravitation  ....  29 

6. — Projecting  the  solar  image 32 

7. — HerschePs  solar  eye-piece     . 33 

8. — Shade  glasses          .       .       .               34 

9. — Polarizing  eye-piece 34 

10. — Rapid  exposing  solar  shutter       .       .       .       .       .       .  35 

11. — Prismatic  refraction  of  light 47 

12. — Prismatic  spectroscope 50 

13.— Diffraction  of  light        ....'....  51 

14. — Plane  grating  spectroscope 53 

15. — Concave  grating  spectroscope 54 

16.— The  150-foot  tower  (Mount  Wilson)          ....  58 

17. — Energy-spectra  of  the  sun  and  the  perfect  radiator         .  69 

18.— Pouillet's  pyrheliometer 73 

19. — Abbot's  silver  disk  pyrheliometer 76 

20. — Angstrom's  pyrheliometer 77 

21. — Angstrom's  pyrheliometer.     Details 77 

22. — Abbot's  water  flow  pyrheliometer 79 

23.— The  bolometer 81 

24. — Holographs  of  the  solar  spectrum 83 

25. — Brightness  on  solar  disk .106 

26. — Energy  spectra  on  solar  disk 109 

xii 


PLATES  AND   ILLUSTRATIONS   IN  TEXT 

FIG.  PAGE 

27. — Huggin's  first  observation  of  a  prominence  in  full  sun- 
shine   143 

28. — Double  reversal  of  the  D-lines 147 

29.— Double  reversal  of  C-line 148 

30. — "  Arrow-head  "  spectrum 148 

31. — Opened  slit  of  the  spectroscope 149 

32 . — Chromosphere  and  prominences  as  seen  in  the  spectrum    .  151 

33. — Relative  frequency  of  protuberances  and  sun-spots      .  153 

34-39. — Eruptive  prominences 157 

40-45. — Quiescent  prominences 159 

46-51. — Forms  of  prominences 163 

52. — Motion  indicated  by  prominence  spectra          .       .        .165 

53. — Sun-spots  and  terrestrial  magnetism 187 

54.' — Sun-spots  and  terrestrial  temperatures  and  magnetism  .  191 

55. — Spoerer's  curves  of  sun-spot  latitude 194 

56.— Solar  diagram  (Young) 216 

57. — Normal  and  anomalous  dispersion 233 

58.— Michelson's  hypothesis 260 

59. — Displacement  diagram 261 

60. — March  of  insolation  (Mount  Wilson)          ....  285 

61. — Holographs  of  the  solar  spectrum 292 

62. — Atmospheric  transmission  plats 294 

63. — Insolation  and  terrestrial  temperatures      .        .        .        .316 
64. — Apparent  variations  of  the  sun's  radiation  (Smithsonian 

observations) 321 

65.    Hypothetical  temperature  diagram 325 

66.— Stomata  (Schwendener) 338 

67. — Promotion  of  carbon  assimilation  by  light,  and  absorp- 
tion of  light         343 

68. — Eneas' solar  engine 367 

69. — Boiler  of  Eneas'  solar  engine 368 

70. — Adams'  solar  cooker 380 

71. — Intensity  of  sun  rays  (Mount  Wilson  and  Washington)  384 
72. — Intensity  of  solar  radiation   (Different  latitudes  and 

elevations) 385 


LIST  OF  TABLES 


TABLE  PAGE 

I. — Principal  data  of  the  solar  system  ....  3 

II. — Velocity  of  light  in  different  media  ....  46 

III. — Principal  solar  spectrum  lines 90 

IV. — Chemical  elements  found  in  the  sun        ...  91 

V. — Chemical  elements  doubtfully  occurring  in  the  sun  92 

VI. — Corrections  to  Rowland's  wave-lengths    ...  96 

VII. — Distribution  of  radiation  over  the  sun's  disk   .        .  107 

VIII. — Energy  spectrum  relations  over  the  sun's  disk   .  110 

IX. — Daily  rotation  of  the  sun's  surface     ....  126 

X. — Measures  of  ninety-two  lines  in  the  flash  spectrum 

near  Hg •      .       .       .       .178 

XI. — Years  of  sun-spot  maxima  and  minima  and  maxi- 
mum intensities 186 

XII. — Hydrogen  spectrum  in  sun-spots      ....  206 

XIII. — Spectrum  lines  affected  in  sun-spots        .       .       .  206 

XIV. — Sun-spot,  hot  arc  and  cool  arc  spectra  .        .       .  209 
XV. — Moissan's  experiments  on  the  vaporization  of  the 

elements  of  the  iron  family     ....  238 

XVI. — Differences  between  observed  and  computed  values 

of  atmospheric  transmission         ....  242 

XVII. — Observed  intensity  of  solar  radiation,  Mount  Wil- 
son, July  6, 1910     .       .        .       .       .       .       .2,87 

XVIII. — The  intensity  of  radiation  in  different  parts  of  the 

solar  spectrum 288 

XIX. — Transmission  for  total  solar  radiation     .        .       .  296 

XX. — Atmospheric  transmission  for  homogeneous  rays  297 

XXI.— Sunlight  and  sky  light  (Exner)         ....  301 

XXII.— Sunlight  and  sky  light  (Roscoe)       ....  302 

xv 


LIST  OF  TABLES 

TABLE  PAGE 

XXIII. — Sky  light  on  a  vertical  surface  (Exner,  Schramm)  303 
XXIV. — Sun  and  sky  light.     Relative  brightness  for  dif- 
ferent wave-lengths  (Mount  Wilson)          .        .  305 
XXV. — Average  brightness  of  sky  zones.      Flint    Island 

and  Mount  Wilson 306 

XXVI.— Ratio  of  total  radiations:  Sky  to  sun       .        .        .307 
XXVII. — Yearly  means   and   mean  daily  temperature  de- 
partures             311 

XXVIII. — Composition  of  food  products 333 

XXIX. — Economy  of  Helianthus  Annuus     ....  361 

XXX. — Percentage  reflecting  powers  of  various  reflectors  3  88 
XXXI. — Classification  of  stellar  spectra          .        .       .       .410 
XXXII. — Intensities  in  stellar  spectra  (Wllsing  and  Schei- 

ner) 411 

XXXIII. — Spectroscopic  binaries.  Spectral  types,  periods  and 

eccentricities  (Campbell) 422 

XXXIV. — Spectral  types  and  velocities  in  space  (Kapteyn)     .  430 


INTKODUCTION 

WE  depend  on  the  sun  for  life,  warmth,  light,  and 
all  mechanical  and  electrical  powers.  Its  constant 
supply  of  heat  is  necessary  to  prevent  the  oceans  and 
even  the  air  from  freezing.  The  supply  of  coal  which 
we  are  now  using  is  but  an  evidence  of  the  sun's  light 
in  former  ages.  To  pass  to  the  enumeration  of  the 
comforts  and  luxuries  and  beauties  we  owe  to  the 
solar  rays  would  lead  us  far  astray,  and  is  indeed 
wholly  unnecessary  because  all  men  acknowledge 
and  many  worship  the  sun  as  the  source  of  these  bene- 
fits. It  would  be  a  gross  neglect  to  omit  the  closer 
investigation  of  such  unique  relations  as  those  the 
sun  maintains  to  life.  Yet  the  study  of  the  means 
of  increasing  the  usefulness  of  the  sun  has  been  ne- 
glected, and  it  is  rather  in  the  investigation  of  its 
curious  features  that  solar  researches  have  gone 
farthest. 

The  enormous  brilliance  and  heat  of  the  solar  rays 
suggestive  of  temperatures  far  above  any  which  can 
be  produced  on  the  earth;  the  marked  dimness  and 
brown  shade  of  the  edge  or  limb  of  the  sun  relative  to 
the  center;  the  fluctuating  march  of  spots  across  the 
disk;  the  variable  rates  of  rotation  of  the  sun's  sur- 
face in  different  latitudes;  the  brilliant  markings 
2  xvii 


INTRODUCTION 

called  faculse  which  accompany  the  spots;  the  weird 
and  highly  beautiful  phenomena  of  total  solar  eclip- 
ses; all  these  have  long  been  the  objects  of  minute 
study.  In  the  last  half  century  the  development  of 
the  spectroscope  has  led  to  great  progress  in  the  more 
intimate  and  satisfactory  knowledge  of  the  sun;  so 
that  we  now  know  many  of  the  chemical  elements  of 
which  it  is  composed;  the  approximate  temperature 
of  its  surface ;  the  motion  of  the  vapors  at  and  near 
the  surface;  the  approximate  pressure  under  which 
they  lie;  the  magnetic  character  and  cyclonic  struc- 
ture of  sun  spots;  their  relative  coolness  as  compared 
with  their  surroundings;  besides  many  other  details 
hardly  to  be  credited  as  known  of  a  body  situated 
nearly  ninety-three  millions  of  miles  away. 

By  bonds  unseen  yet  altogether  stronger  than  a 
bar  of  steel  thousands  of  miles  in  diameter  the  sun 
holds  to  itself  the  moving  family  called  the  solar 
system,  comprising  the  earth  and  moon,  the  seven 
other  great  planets  with  their  satellites,  half  a  thou- 
sand asteroids,  or  minor  planets,  besides  numerous 
comets  and  meteorites.  It  has  required  the  lifelong 
labors  of  many  men  of  exceptional  genius,  like  New- 
ton, coupled  with  centuries  of  no  less  praiseworthy  if 
less  brilliant  accumulations  of  accurate  observations 
to  have  given  us  the  full  knowledge  which  we  now 
enjoy  of  the  distances,  dimensions,  masses  and  orbits 
of  the  solar  system. 

Although  the  distance  from  the  sun  to  the  orbit  of 
Neptune  is  2,800,000,000  miles,  the  solar  system  is 

xviii 


INTRODUCTION 

but  a  speck  in  the  vast  universe  of  the  stars.  In  the 
year  1901  there  flared  up  in  the  constellation  Perseus 
a  new  star  which  for  a  few  days  rivaled  the  brightest 
stars  of  the  heavens  in  its  brilliancy,  and  then  slowly 
faded  away  into  insignificance.  To  dwellers  on  the 
earth  this  sight  was  new  in  1901,  but  in  reality  that 
new  star  was  so  far  away  that  its  sudden  burst  of 
light,  traveling  toward  us  186,000  miles  a  second, 
had  been  on  the  journey  since  the  days  of  Cromwell, 
and  the  star  had  faded  away  nearly  three  centuries 
ago.  At  such  enormous  distances  are  the  stars  that, 
although  some  of  them  are  believed  to  be  millions  of 
miles  in  diameter,  they  present  no  real  disks  even  in 
the  largest  telescopes,  so  that  the  details  of  their  sur- 
faces cannot  be  examined.  Nevertheless,  by  the 
powerful  aid  of  the  spectroscope,  much  is  known  of 
the  chemical  constitutions  of  the  stars;  and  many  of 
them  have  been  shown  to  belong  to  revolving  sys- 
tems, the  components  of  which,  though  separated  in 
some  instances  by  greater  distances  than  is  Jupiter 
from  the  sun,  are  separately  indistinguishable  to  the 
telescope.  Still  we  might  despair  of  knowing  much 
more  of  the  physics  of  the  stars  if  it  were  not  that  the 
spectroscope  shows  also  that  the  sun  is  but  one  of 
them  close  by,  and  that  a  large  class  among  the  stars 
is  probably  in  a  similar  condition  as  to  temperature, 
and  made  up  of  the  same  chemical  substances  as  the 
sun. 

Both   telescopic   and   spectroscopic   observations 
have  shown  that  the  solar  system  is  moving  at  a  rapid 

xix 


INTRODUCTION 

rate  towards  the  constellation  Hercules,  although  no 
change  in  the  aspect  of  the  heavens  sensible  to  the 
naked  eye  would  accumulate  for  an  immense  period 
of  years.  By  means  of  the  displacement  of  the  earth 
in  its  orbit  around  the  sun,  amounting  to  over  180 
millions  of  miles,  semi-annually,  it  has  been  possible 
to  obtain  with  fair  accuracy  the  distances  of  many  of 
the  individual  stars,  and  from  these  by  statistical 
methods  to  go  further  and  estimate  the  average  dis- 
tances of  all  the  stars  of  a  given  brightness.  These 
various  examples  indicate  the  important  place  of  the 
sun  in  stellar  investigations;  and  indeed  the  study 
of  the  sun  as  a  typical  star,  though  quite  recently 
developed,  seems  bound  to  throw  much  more  light 
on  the  subject  of  the  nature  of  the  universe. 

Considering  the  sun  as  the  fountain  of  light  and 
heat  upon  the  earth,  perhaps  the  first  question  which 
suggests  itself  is  this:  How  much  radiant  energy 
reaches  the  earth  from  the  sun  in  a  given  time?  This 
utilitarian  branch  of  solar  investigation  has  been 
comparatively  neglected.  No  more  striking  proof  of 
the  neglect  need  be  cited  than  to  say  that  the  fore- 
most text-book  on  meteorology,  published  since  1900, 
states  various  determinations  of  the  intensity  of 
solar  radiation  at  the  earth's  mean  distance  which 
range  from  1.76  to  4.06  calories  per  square  centi- 
meter per  minute.  Of  these  the  author  of  the  text- 
book prefers  one  which  is  confessedly  the  mean  of 
such  divergent  numbers  as  2.63  and  3.50,  one  of 
which  numbers  was  thought  by  its  originator  to  be 

xx 


INTRODUCTION 

too  low,  and  the  other  too  high!  As  will  be  shown 
later,  there  can  be  little  question  now  (1910)  that 
the  true  value  is  about  1.95  calories;  but  how  remark- 
able it  is  that  one  of  the  fundamental  constants  of 
Nature  should  have  been  uncertain  within  such  wide 
limits,  so  late  as  the  beginning  of  the  twentieth  cen- 
tury. Imagine,  for  analogy,  that  it  had  been  stated 
in  a  standard  work  on  astronomy,  published  in  1905, 
that  the  sun's  distance  (which  is  of  no  greater  im- 
portance than  the  constant  of  radiation)  might  be 
anything  between  80  millions  and  200  millions  of 
miles  so  far  as  known,  and  that  it  was  generally  sup- 
posed to  be  140  millions! 

Some  of  the  more  important  questions  connected 
with  the  sun's  action  as  the  fountain  of  light  and  heat 
are  the  following:  Is  the  solar  radiation  uniform  or 
variable?  What  losses  does  it  suffer  in  the  earth's 
atmosphere?  Are  there  changes  of  transparency  in 
the  sun's  outer  layers  sufficient  to  alter  the  earth's 
supply  of  radiation  appreciably?  How  much  solar 
radiation  does  the  earth  reflect,  unused,  to  space? 
How  does  the  earth's  temperature  depend  on  solar 
radiation  and  on  the  transparency  of  the  air?  If 
there  should  be  variations  of  solar  radiation,  how 
great  changes  of  temperature  of  different  stations 
on  the  surface  of  the  earth  ought  to  follow,  and  how 
long  would  such  responses  be  delayed?  In  short, 
are  solar  studies  applicable  to  weather  prediction? 
What  methods,  if  any,  can  be  economically  used  to 
store  and  employ  the  sun's  energy  for  power  or  heat- 

xxi 


INTRODUCTION 

ing?  What  influences  do  changes  in  the  intensity 
or  color  of  the  light  falling  on  different  plants  produce 
on  their  growth  and  fruitage?  May  advantageous 
variations  of  plants  be  promoted  by  the  control  of 
their  radiation  supply?  What  can  be  done  with 
solar  rays  for  the  promotion  of  health? 

In  the  pages  which  follow  the  sun  will  be  considered 
in  these  three  aspects:  First  as  the  controlling  mem- 
ber of  the  solar  system;  second,  as  an  object  of  in- 
quiry, interesting  in  itself,  but  still  more  so  as  the 
nearest  star,  and  typical  of  a  large  class  of  stars; 
third,  as  the  fountain  of  light  and  heat,  and  through 
them  of  life  on  the  earth.  It  is  indispensable  to  any 
satisfactory  understanding  of  the  second  and  third 
branches  to  be  familiar  with  the  methods  and  princi- 
ples which  are  now  being  employed  in  solar  investi- 
gations. For  the  convenience  of  the  reader  a  general 
account  of  these  is  given  in  Chapter  II,  which  there- 
fore has  to  do  directly  with  physics,  and  only  sec- 
ondarily with  the  sun,  but  which  forms  the  ground- 
work of  the  chapters  relating  directly  to  solar  phe- 
nomena. Illustrated  descriptions  of  some  of  the 
instruments  used  in  solar  research  will  also  be  found 
in  Chapter  II,  and  appropriate  references  to  these 
descriptions  and  to  the  statements  of  the  general 
relations  will  be  found  in  the  text  of  subsequent 
chapters. 

To  avoid  premature  discussion,  the  various  solar 
phenomena  will  be  described  first  without  much 
attention  to  their  explanation,  except  as  seems  nec- 

xxii 


INTRODUCTION 

essary  to  fix  attention  upon  significant  facts.  Solar 
theories  are  dealt  with  in  Chapter  VI.  One  principal 
exception  to  this  course  is  in  the  frequent  noting  of 
applications  of  Kirchhoff  s  discoveries  on  the  rela- 
tions of  temperature,  radiation,  and  absorption.  It 
may  be  that  future  writers  on  the  sun  will  attribute 
to  anomalous  dispersion  many  of  the  phenomena 
here  set  down  as  due  to  absorption  and  motion.  But 
although  anomalous  dispersion  hypotheses  are  so 
strongly  advocated  by  Julius,  the  writer  feels  confi- 
dent that  his  own  preference  for  the  older  views  is 
still  shared  by  most  students  of  solar  physics. 

What,  after  all,  is  the  sun,  and  how  can  we  best 
explain  the  principal  solar  phenomena?  No  doubt 
many  will  find  the  views  here  advanced  heretical, 
but  for  the  writer  the  existence  of  the  cloudy  photo- 
sphere, so  firmly  believed  in  by  most  solar  observers, 
seems  so  highly  improbable  that  he  has  ventured  to 
advocate  the  view  of  a  purely  gaseous  sun.  But  in 
doing  so  it  is  not  Schmidt's  refraction  theory  to 
which  he  turns  to  explain  the  sharp  solar  boundary. 
According  to  Lord  Rayleigh  our  own  atmosphere, 
if  freed  from  dust,  would  still  scatter  light  by  the 
action  of  the  gases  themselves.  Schuster  and  Natan- 
son  have  computed  this  effect,  independently,  and 
both  find  that  the  purely  gaseous  scattering  goes  far 
to  explain  in  full  the  observed  weakening  of  the  direct 
beam  of  the  sun  above  Mount  Wilson  for  rays 
which  are  not  selectively  absorbed.  This  weakening 
amounts  to  several  per  cent.  If  then  the  gases  of 

xxiii 


INTRODUCTION 

the  atmosphere  of  the  earth,  which  extend,  in  density 
sufficient  to  scatter  light  appreciably,  perhaps  only 
fifty  miles  in  altitude,  suffice  to  scatter  several  per 
cent,  of  a  beam  of  light,  it  seems  probable  that  we 
can  see  at  the  most  not  more  than  a  very  few  thou- 
sand miles  into  the  gaseous  body  of  the  sun,  which,  at 
the  layer  producing  the  Fraunhofer  lines,  seems  to 
be  under  several  atmospheres  of  pressure.  Admit- 
ting this,  how  deep,  measured  radially,  can  one  see 
near  the  sun's  edge,  where  the  few  thousand  miles 
above  mentioned  will  lie  along  a  line  of  sight  nearly 
tangent  to  the  sun?  It  would-seem  that  at  the  sun's 
edge  a  shell  of  gas  of  only  a  few  hundred  miles  in 
thickness  must  suffice  to  fully  veil  all  that  lies  below. 
Viewed  from  the  earth  this  would  correspond  to  a 
fraction  of  a  second  of  arc,  so  that  a  gaseous  sun  at 
93,000,000  miles  away  would  present  an  apparently 
sharp  boundary.  From  these  considerations  depend 
various  consequences  adapted  to  the  explanation  of 
solar  phenomena.  To  be  sure  there  are  several 
apparently  powerful  objections  to  this  view  of  a 
purely  gaseous  sun,  but  they  seem  not  to  be  in- 
superable. 

My  good  friend,  Prof.  J.  C.  Kapteyn,  has  en- 
couraged me  to  set  down  several  hypotheses  which 
can  be  regarded  as  only  slenderly  founded.  Among 
these  are  the  hypotheses  of  the  causes  of  some  strange 
phenomena  of  geological  climates,  touched  upon  in 
Chapter  VI,  and  more  fully  discussed  in  Chapter 
VII ;  the  hypotheses  of  the  causes  of  some  peculiar- 

xxiv 


INTRODUCTION 

ities  of  stellar  evolution  given  in  Chapter  X;  and 
as  some  readers  may  be  disposed  to  think,  even  the 
explanation  of  solar  phenomena  already  mentioned 
which  occupies  a  large  part  of  Chapter  VI.  Pro- 
fessor Kapteyn  is  of  the  opinion  that  a  bushel  of  chaff 
is  worth  searching  by  a  Crusoe  if  it  contains  some 
grains  of  corn  that  will  sprout,  and  so  my  defense 
for  my  temerity  in  including  such  speculations  is  that 
they  may  interest  some  readers  to  begin  some  more 
fruitful  researches. 


THE   SUN 


CHAPTER  I 

THE      SOLAR     SYSTEM.      THE      SUN'S      DISTANCE.       ITS 
DIMENSIONS 

THE  objects  which  appear  to  move  among  the  stars, 
namely  the  sun,  planets,  minor  planets,  moons,  me- 
teors and  comets,1  compose  the  solar  system.  For- 
merly it  was  believed  that  all  the  heavenly  bodies 
revolve  about  the  earth.  But  now  the  theory  of 
Copernicus  is  fully  verified,  arid  the  earth  is  known  to 
be  only  a  planet,  of  much  smaller  size  than  Jupiter, 
Saturn,  Uranus  or  Neptune,  though  larger  than  Mars, 
Venus  or  Mercury,  and  like  the  other  planets  it  re- 
volves about  the  sun.  Galileo  was  threatened  with 
torture  and  forced  to  perjure  himself  because  he  be- 
lieved this,  which  shows  how  fortunate  we  are  to  live 
in  the  present  age. 

The  moon  has  actually  only  -—  as  great  a  diameter 

4UU 

as  the  sun,  although  they  appear  to  be  about  equal. 
It  is  the  immense  distance  by  which  the  sun  and 
planets  are  separated  from  the  earth  in  comparison 
with  the  distances  we  are  accustomed  to  travel  over 

1  Not  all  the  comets  remain  permanently  attached  to  the  solar 
system,  but  many  of  them  do. 

1 


THE  SUN 

d  with  the  distance  of  the  moon, 
which  prevents  us  from  immediately  realizing  the 
great  bulks  of  these  distant  bodies.  In  the  following 
table  is  a  summary  of  the  approximate  dimensions 
and  principal  characteristics  of  the  larger  members 
of  the  solar  system.  The  means  of  determining  the 
sun's  distance,  dimensions  and  rotation  will  be  given 
later. 

GRAVITATION 

Accustomed  as  we  are  to  regard  inches  and  feet 
as  ordinary,  miles  as  considerable,  and  thousands  of 
miles  as  very  great  distances,  it  may  seem  almost  in- 
credible that  there  should  be  any  bond  between  the 
the  sun  and  Neptune,  situated  as  they  are  2,800,000,- 
000  miles  apart.  There  is,  however,  a  bond  between 
them  so  strong  that  it  would  require  the  strength  of  a 
bar  of  steel  500  miles  in  diameter  to  take  its  place 
in  preventing  the  escape  of  Neptune  from  the  sun. 
This  bond  we  call  gravitation.  Every  body  in  the 
universe  is  believed  to  attract  every  other  body  in 
the  universe  with  a  force  proportional  to  the  mass  or 
quantity  of  matter  the  body  contains,  and  inversely 
proportional  to  the  square  of  the  distance  between 
their  centers  of  gravity.  On  the  one  hand,  this  law 
of  gravitation  applies  between  all  bodies  on  the  earth 
as  well  as  between  the  earth  itself  and  any  one  of 
them;  and,  on  the  other  hand,  there  is  evidence  that 
it  holds  also  among  the  fixed  stars.  The  weight  of  a 
stone  is  the  measure  of  the  attraction  between  it  and 

2 


THE  SOLAR  SYSTEM 


•  " 


|J 


§1 

+j  * 

fl  S 
£.5 


O   O   T-H    O  ~~^<N    T-H    O   O   T-4 


^      S 


CO  t^  C<1  TH 


CO  !*-• 

COOI-C 

i  p-  i— i  O  O  C 


iO 


800O< 
i-H  CO  I 

'  t^O5<N  • 


10  co  t>-  r^  -f 


CO  O  T-H  -«HH  <N 

oo  t^  co  co 


O  O  l-»  O5  iC 

-»O<N    •    •  '-H 


T-H  -HH  O5        •  CO  O 


Ps 


c  °  S 

i-sj 


sa 

coco 

1C  CO 


02 

1^       OiO 

CO  T—  ( 


a  a    a  a 

CO  !>•        O  ^f 

IIOCO  »Or-H 


CO 

>0 


T-HTtl    OOO 


' 
i—  ii-*coOcOO'—  i  OS  '-'  i—  ' 

iOOOdr-lrHi-lOO'-l 


COt^fMi-Ht^COCOi-Hi—  < 
COcOOSr^iOOOOOOOOiOO 


r^iOOO 

rH  C^  -* 


1>-  t^ 


«  ^ 


THE  SUN 

the  earth,  and  with  a  sufficiently  sensitive  apparatus 
the  attraction  of  two  stones  for  each  other  may  be 
clearly  shown.  If  weighed  with  a  sufficiently  deli- 
cate spring  balance,  a  weight  will  be  found  lighter  at 
the  top  of  a  mountain  than  at  its  base,  in  the  ratio 
of  the  square  of  the  distances  of  the  earth's  center  at 
the  two  places  of  observation.  There  are  chemical 
balances  so  delicate  that  an  object  appears  to  weigh 
differently  accordingly  as  the  weights  are  placed  side 
by  side  or  one  on  the  top  of  the  other,  and  this  is 
because  of  the  difference  in  distance  from  the  weights 
to  the  earth's  center  in  the  two  cases. 

The  attraction  of  gravitation  between  the  sun 
and  Neptune  amounts  to  8  x  1016  (8  followed  by 
sixteen  ciphers)  tons.  If  there  was  nothing  opposed 
to  this  force,  Neptune  obviously  would  fall  into  the 
sun.  It  is  the  motion  of  the  planet  in  its  orbit,  at 
right  angles  to  the  line  leading  towards  the  sun,  which 
maintains  the  distance  between  them. 

Kepler's  laws  of  planetary  motion  are  as  follows: 

I.  The  orbit  of  each -planet  is  an  ellipse,  with  the 
sun  in  one  of  its  foci. 

II.  The  radius  vector  of  each  planet  describes  equal 
areas  in  equal  times. 

III.  The  squares  of  the  periods  of  revolution  of  the 
planets  are  proportional  to  the  cubes  of  their  mean 
distances  from  the  sun. 

There  is  but  little  difference  between  the  major  and 
minor  axes  of  the  elliptical  orbits  of  most  of  the  plan- 
ets, or,  in  other  words,  their  orbits  are  nearly  circular. 

4 


THE  SOLAR  SYSTEM 

But  this  is  not  true  of  the  orbits  of  Mercury  and  Mars, 
as  is  shown  in  Table  1. 

It  may  be  surprising  to  some  readers  that  Kepler 
could  have  possessed  so  much  knowledge  of  the  dis- 
tances of  the  planets  from  the  sun  as  to  enable  him  to 
verify  the  third  law,  while  as  yet  the  actual  distances 
in  miles  were  not  even  roughly  known.  But  only 
the  ratios  of  the  distances  of  the  planets  were  re- 
quired by  Kepler,  and  these  could  be  fixed  inde- 
pendently of  the  actual  distances  by  the  following 
method,  known  since  the  days  of  Hipparchus,  which 
I  quote  from  Young's  "The  Sun." 

"  First,  observe  the  date  when  the  planet  comes 
to  its  opposition,  i.e.,  when  sun,  earth,  and  planet 
are  in  line,  as  in  the  fig- 
ure, where  the  planet 
and  earth  are  repre- 
sented by  M  and  E. 
Next,  after  a  known 
number  of  days,  say 
one  hundred,  when  the 

planet  has  advanced  to  M'and  the  earth  to  E',  observe 
the  planet's  elongation  from  the  sun,  i.e.,  the  angle 
M'E'S.  Now,  since  we  know  the  periodic  times  of 
both  the  earth  and  planet,  we  shall  know  both  the 
angle  MSM'  moved  over  by  the  planet  in  one  hun- 
dred days,  and  also  ESE',  described  in  the  same  time 
by  the  earth.  The  difference  is  M'SE,  often  called 
the  synodic  angle.  We  have,  therefore,  in  the  tri- 
angle M'SE',  the  angle  at  E'  measured,  and  the  angle 

5 


THE   SUN 

M'SE'  known  as  stated  above,  and  hence  by  the 
ordinary  processes  of  trigonometry  we  can  find  the 
relative  values  of  its  three  sides." 

Thus,  by  means  of  comparatively  simple  astronom- 
ical observations,  all  the  relative  distances  in  the 
solar  system  can  be  fixed  with  high  accuracy.  It  is  a 
work  of  far  greater  difficulty,  as  we  shall  see,  to  meas- 
ure any  of  these  distances  absolutely. 

Kepler's  three  laws  were  known  before  the  year 
1620,  but  without  explanation.  Sir  Isaac  Newton 
discovered,  about  the  year  1679,  that  all  three  laws 
are  direct  consequences  of  the  laws  of  motion,  pro- 
viding it  is  assumed  that  all  bodies  attract  one  an- 
other with  a  force  varying  inversely  as  the  square  of 
the  distance.  This  latter  principle  is  Newton's  law 
of  gravitation. 

At  the  present  day  no  well-informed  person  ques- 
tions either  the  Copernican  system  or  the  universal 
sway  of  gravitation ;  for,  while  not  every  such  person 
has  the  mathematical  knowledge  requisite  to  examine 
all  the  proofs  of  these  fundamental  facts,  he  yet  feels 
entire  faith  in  the  conclusions  unanimously  agreed 
upon  by  such  masters  as  Kepler,  Newton,  Laplace, 
and  many  others  of  scarcely  less  renown,  who  have 
overcome  the  tremendous  mathematical  difficulties 
in  which  the  knowledge  of  the  motions  of  the  solar 
system  is  involved.  Every  planet  and  satellite  at- 
tracts every  other,  and  perturbs  its  motion  from  the 
simple  orbit  which  would  exist  if  there  were  only  two 
bodies  concerned.  In  a  large  astronomical  library 

6 


THE  SOLAR  SYSTEM 

one  may  find  printed  in  a  quarto  or  folio  volume  the 
final  equation  representing  the  motion,  for  instance, 
of  the  moon.  Such  an  equation  comprises  line  upon 
line  and  page  after  page,  including  thousands  of 
terms  required  to  account  for  all  the  disturbing  fac- 
tors. None  but  a  master  can  handle  such  a  problem. 

Prof.  E.  W.  Brown,  writing  in  1904  l  of  his  inves- 
tigation of  the  theory  of  the  moon's  motion,  says: 

"  A  few  brief  details  about  the  amount  of  time  and 
labour  expended  may  not  be  uninteresting.  From 
1890  to  1895  certain  classes  of  inequalities  were  cal- 
culated, but  the  work  was  only  begun  on  a  systematic 
plan,  which  involved  a  fresh  computation  of  all  the 
inequalities  previously  found,  at  the  beginning  of 
1896.  Mr.  Sterner  began  work  for  me  in  the  autumn 
of  1897  and  finished  it  in  the  spring  of  the  present 
year,  though  neither  of  us  was  by  any  means  contin- 
uously engaged  in  calculation  during  that  period. 
He  spent  on  it,  according  to  a  carefully  kept  record, 
nearly  three  thousand  hours,  and  I  estimate  my  share 
as  some  five  or  six  thousand  hours,  so  that  the  calcu- 
lations have  probably  occupied  altogether  about  eight 
or  nine  thousand  hours.  There  were  about  13,000 
multiplications  of  series  made,  containing  some  400,- 
000  separate  products;  the  whole  of  the  work  re- 
quired the  writing  of  between  four  and  five  millions 
of  digits  and  plus  and  minus  signs.  Although  the 
problem  now  completed  constitutes  by  far  the  longer 

1  Royal  Ast.  Soc.  Monthly  Notices,  vol.  Ixv,  p.  107,  1904? 
3  7 


THE  SUN 

part  of  the  whole,  much  remains  to  be  done  before 
it  is  advisable  to  proceed  to  the  construction  of 
tables." 

Every  large  scientific  institution  or  observatory 
has  almost  daily  communications  from  persons  of 
very  moderate  attainments  who  presume  to  question, 
nay  rather  to  spurn,  the  most  well-attested  facts  of 
human  knowledge.  Such  persons  seem  to  prefer 
especially  to  direct  their  attacks  on  the  following 
facts:  the  Copernican  system;  the  law  of  universal 
gravitation;  the  first  and  second  laws  of  energy; 
and,  finally,  the  high  temperature  of  the  sun.  No 
argument  can  refute  them,  because  they  have  not 
the  requisite  learning  to  comprehend  it,  which  is  no 
disgrace,  but  which  should  make  men  modest  enough 
to  have  faith  in  those  who  excel  them  immeasurably. 
Hence  it  is  the  policy  of  most  scientific  institutions  to 
avoid  entirely  discussions  of  these  subjects  with  such 
correspondents. 

Professor  Newcomb  tells,  in  his  "  Reminiscences 
of  an  Astronomer,"  of  such  a  critic  who  called  upon 
him  and  announced  his  disbelief  in  Sir  Isaac  Newton's 
theory  of  gravitation.  Professor  Newcomb  proposed 
to  the  skeptic  that  he  jump  out  of  the  window  and 
convince  himself  of  the  existence  of  gravitation. 
Being  thus  pressed,  the  visitor  stated  that  he  be- 
lieved that  gravitation  extended  no  further  than  the 
air,  -and  did  not  go  up  to  the  moon.  Professor  New- 
comb  asked  him  if  he  had  ever  been  there  to  see,  and 
when  his  caller  answered  "No, "  replied  that,  until 

8 


THE  SUN'S  DISTANCE 

one  of  them  could  go  to  the  moon  and  try  the  experi- 
ment, he  doubted  if  they  could  ever  agree! 

THE  SUN'S  DISTANCE 
(1)  Geometrical  Methods. 

Since  the  ratios  of  the  distances  between  all  the 
principal  members  of  the  solar  system  can  be  fixed 
with  great  accuracy  by  ordinary  astronomical  obser- 
vations, it  suffices  to  measure  accurately  in  miles  or 
kilometers  the  distance  from  the  earth  to  the  sun  or 
to  any  one  of  the  planets,  and  this  fixes  the  scale  of 
the  whole  system.  The  great  astronomical  unit  is 
the  mean  distance  from  the  earth  to  the  sun,  and 
many  determinations  of  it  have  been  made  in  the  last 
250  years.  Still  astronomers  are  not  quite  content, 
although  there  is  no  doubt  that  we  know  the  distance 

to  within  — —  part  of  itself.    To  avoid  using  many 
lUUU 

figures,  it  is  customary  to  speak  of  the  sun's  " paral- 
lax" instead  of  its  distance.  The  parallax  is  the 
angle  which  the  earth's  equatorial  radius  would  sub- 
tend if  viewed  from  the  center  of  the  sun  at  the  mean 
solar  distance.  This  angle  is  nearly  8.80  seconds  of 
arc,  or  about  0.000044  in  circular  measure.  In  other 
words,  the  earth's  mean  radius  is  0.000044  of  the 
sun's  mean  distance,  and  the  latter  is  about  92,900,- 
000  miles. 

Since  the  solar  parallax  is  so  small,  the  usual 
method  of  surveyors  for  finding  the  distance  of  an 
inaccessible  object  is  not  applicable  here.  For,,  if 

9 


THE  SUN 

the  earth's  radius  of  4,000  miles  subtends  only  8.8", 
no  two  stations  on  the  earth's  surface  which  can 
be  seen  from  each  other  could  possibly  be  far  enough 
apart  to  serve  as  a  suitable  base  line  for  a  solar 
triangulation.  Fortunately  the  observer  can  avail 
himself  of  the  fixed  stars  in  the  investigation.  These 
may  be  regarded  as  an  infinitely  great  distance  away, 
and  an  apparent  displacement. of  objects  in  the  solar 


FIG.  2. 

system  among  the  stars  may  be  observed  from  two 
stations  at  opposite  sides  of  the  earth,  or  at  the 
same  station  by  two  observations  several  hours 
apart.  The  following  explanation  of  this  parallax 
method  is  quoted  from  Young's  "The  Sun." 

"Fig.  2  illustrates  the  method  of  observation. 
Suppose  two  observers,  situated,  one  near  the  north 
pole  of  the  earth,  the  other  near  the  south.  Looking 

10 


THE  SUN'S  DISTANCE 

at  the  planet,  the  northern  observer  will  see  it  at  N 
(in  the  upper  figure) ,  while  the  other  will  see  it  at  S, 
farther  north  in  the  sky.  If  the  northern  observer 
sees  it  as  at  A  (in  the  lower  part  of  the  figure),  the 
southern  will  at  the  same  time  see  it  as  at  B;  and, 
by  measuring  carefully  at  each  station  the  apparent 
distance  of  the  planet  from  several  of  the  little  stars 
(a,  b,  c)  which  appear  in  the  field  of  view,  the  amount 
of  the  displacement  can  be  accurately  ascertained. 
The  figure  is  drawn  to  scale.  The  circle  E  being 
taken  to  represent  the  size  of  the  earth  as  seen  from 
Mars  when  nearest  us,  the  black  disk  represents  the 
apparent  size  of  the  planet  on  the  same  scale,  and  the 
distance  between  the  points  N  and  S,  in  either  figure 
A  or  B,  represents,  on  the  same  scale,  also,  the  dis- 
placement which  would  be  produced  in  the  planet's 
position  by  the  transference  of  the  observer  from 
Washington  to  Santiago,  or  vice  versa." 

Dr.  Gill,  lately  the  Astronomer  Royal  at  the  Cape 
of  Good  Hope,  has  made  many  highly  accurate  meas- 
urements by  this  method.  He  observed  the  oppo- 
sition of  Mars  in  1877  at  Ascension  Island,  employing 
for  his  measurements  a  heliometer  loaned  by  Lord 
Lindsay.  This  instrument  is  a  telescope  having  its 
lens  cut  in  halves,  and  having  a  micrometer  screw  for 
sliding  the  parts  with  reference  to  each  other,  thus 
enabling  the  observer  to  make  the  images  of  two 
stars  formed  by  the  two  halves  to  coincide.  This 
device  is  the  most  accurate  one  known  for  measuring* 
small  angular  displacements  between  stars.  Dr.  Gill 

11 


THE  SUN 

determined  the  displacement  of  Mars  among  the 
stars  as  measured  by  evening  and  morning  observa- 
tions, and  continued  the  work  for  several  weeks.  From 
these  measurements  he  obtained  8.780"±0.020"  for 
the  sun's  parallax. 

Several  of  the  minor  planets  or  asteroids,  though  at 
greater  distances  from  the  earth,  have  proved  more 
favorable  objects  for  these  measurements  than  Mars. 
Being  smaller  and  not  so  highly  colored,  it  appears 
that  more  accurate  measurements  of  their  projections 
among  the  stars  can  be  made.  In  1889  and  1890  a 
concerted  system  of  observations  was  made  upon  the 
asteroids  Victoria,  Iris,  and  Sappho  by  Dr.  Gill,  Dr. 
Elkin  of  the  Yale  College  Observatory,  and  several 
German  observers.  Their  results  range  from  8.796" 
to  8.825",  and  their  mean  is  8.807"±0.006". 

The  discovery  of  the  minor  planet  Eros,  in  1898, 
furnished  an  object  so  much  more  favorable  than  any 
other  for  parallax  determinations  by  this  method  that 
a  great  parallax  campaign  was  lately  carried  through 
upon  this  asteroid  by  many  of  the  leading  observa- 
tories. Undoubtedly  a  still  more  thorough  one  will 
be  undertaken  in  1931.  Eros  has  a  very  elliptical 
orbit ;  so  much  so  that  when  nearest  the  earth  in  the 
most  favorable  oppositions  its  distance  is  only  13,- 
500,000  miles,  and  its  parallax  then  becomes  as  great 
as  60";  while  at  its  most  unfavorable  oppositions  its 
nearest  distance  is  74,000,000  miles  and  its  parallax 
only  11".  In  the  opposition  of  1900-1,  its  nearest 
approach  was  30,000,000  miles,  but  in  1931  it  will  be 

12 


THE  SUN'S   DISTANCE 

within  half  that  distance.  Prof.  Arthur  Hinks 
has  lately  completed  and  published  the  reduction  of 
the  photographic  measurements  of  the  1900-1  in- 
ternational Eros  campaign,  and  he  obtains  the  solar 
parallax  as  8.807"  ±0.0027". 

The  method  just  described  for  fixing  the  scale  of 
the  solar  system  is  generally  believed  to.be  the  best  of 
all  at  present.  But  there  are  several  other  methods 
which  deserve  mention,  and  first,  on  account  of  its 
historical  interest,  the  method  of  the  transit  of  Venus. 
This  planet  as  a  dark  spot  passes  occasionally  be- 
tween us  and  the  sun's  disk.  The  transits  occur  in 
pairs,  about  eight  years  apart,  and  the  pairs  occur 
only  about  once  a  century.  Those  of  June,  1761  and 
1769,  and  of  December,  1874  and  1882,  were  all  ob- 
served with  great  attention  by  astronomers  of  sev- 
eral nations,  for  they  were  regarded,  until  very 
recently,  as  yielding  by  far  the  best  means  of  fixing 
the  solar  parallax.  All  the  methods  used  depend,  of 
course,  on  the  displacement  of  the  planet  on  the  sun's 
disk,  as  viewed  from  opposite  sides  of  the  earth. 

In  Weld's  "  History  of  the  Royal  Society  of  Great 
Britain, "  may  be  found  several  quaint  items  relating 
to  the  transits  of  1761  and  1769.  We,  of  America, 
sometimes  get  the  impression  in  our  early  school  days 
that  King  George  III  was  only  a  crazy  old  despot, 
and  it  will  be  a  satisfaction  to  many  of  us  to  know 
that  he  was  a  very  liberal  patron  of  the  best  scientific 
enterprises.  At  the  request  of  the  Royal  Society  he 
ordered  £1,800  to  be  appropriated  for  the  observa- 

13 


THE  SUN 

tion  of  the  transit  of  1761,  and  in  addition,  the  Ad- 
miralty directed  a  ship  of  war,  the  Sea-Horse,  to 
convey  the  observers  to  Bencoolen  in  India.  The 
Rev.  Nevil  Maskelyne,  afterwards  Astronomer  Royal, 
was  sent  to  observe  at  Saint  Helena. 

The  two  observers  sent  to  Bencoolen  were  Mason 
and  Dixon,  the  names  so  famous  in  American  history 
on  account  of  their  survey  of  "  Mason  and  Dixon's 
Line,"  which  afterwards  led  to  the  popular  name  of 
" Dixie"  for  the  South-land.  Their  ship,  the  Sea- 
Horse,  engaged  a  French  frigate  almost  at  the  shores 
of  England.  The  stands  for  instruments  were  dam- 
aged by  shots,  and  the  observers  could  hardly  be  in- 
duced to  reundertake  the  journey.  On  account  of 
the  delay  thus  occasioned,  they  observed  at  the  Cape 
of  Good  Hope,  instead  of  proceeding  to  India. 

For  the  transit  of  1769  the  King,  at  the  Memorial 
of  the  Royal  Society,  provided  even  more  liberally. 
He  ordered  £4,000  clear  of  fees  to  be  paid  over,  and 
that  any  balance  which  might  be  unexpended  should 
be  for  the  use  of  the  Society.  In  addition,  the  ship 
Endeavor,  under  the  command  of  Lieutenant,  after- 
wards the  famous  Captain,  James  Cook,  was  ordered 
to  the  Pacific  Ocean  to  take  part  in  the  observations. 
Cook  observed  the  transit  successfully  at  what  is  now 
called  Venus  Point,  on  the  island  of  Tahiti.  The 
Royal  Society  sent  observers  also  to  Hudson's  Bay 
and  to  India  on  this  occasion. 

In  1874  and  1882  very  elaborate  preparations  were 
made  by  the  governments  and  private  astronomers  of 

14 


THE  SUN'S  DISTANCE 

many  countries,  including  our  own.  Observations 
were  made  all  over  the  world  and  with  many  kinds  of 
apparatus,  including  heliometers,  micrometers,  and 
photographic  apparatus.  Many  thousands  of  ob- 
servations were  made. 

The  results  of  the  several  transits  of  Venus  are  on 
the  whole  disappointing.  A  general  discussion  of  the 
observations  of  1761  and  1769  was  made  by  Encke  in 
1822,  and  he  found  the  solar  parallax  8.5776".  More 
recent  recomputations  have  shown  that  the  transit  of 
1769  may  be  said  to  indicate  a  parallax  of  between 
8.7"  and  8.9".  From  the  transits  of  1874  and  1882 
different  astronomers  have  computed  widely  different 
results,  ranging  from  8.89"  down  to  8.75".  Newcomb 
adopts  8.794"0  ±  .022". 

(2)  The  Gravitation  Methods. 

Thus  far  we  have  considered  only  geometrical 
methods  for  determining  the  parallax,  and  now  we 
may  notice  another  class  of  quite  a  different  charac- 
ter— the  gravitational  methods,  so  called,  which  de- 
pend on  noting  the  perturbing  effects  of  the  different 
planets  and  satellites  on  one  another.  One  of  the 
best  of  them  depends  on  observations  of  the  motion 
of  the  moon.  It  was  Hansen's  studies  of  the  moon's 
parallactic  inequality  which  led  him  to  announce,  in 
1854,  the  inadmissibility  of  Encke's  value,  8.5776", 
for  the  solar  parallax.  The  perturbation  of  the 
moon's  orbit  by  the  sun  causes  the  interval  from  new 
moon  to  first  quarter  to  be  about  eight  minutes  longer 

15 


THE  SUN 

than  that  from  the  quarter  to  full  moon.  This  in- 
equality depends  on  the  ratio  of  the  radii  of  the  orbits 
of  the  moon  and  the  earth.  Hence,  as  the  moon's  dis- 
tance is  known,  the  solar  parallax  could  be  determined 
if  the  inequality  could  be  exactly  measured.  New- 
comb  gives  8.794"  as  the  most  probable  result  thus 
obtained,  but  Prof.  E.  W.  Brown's  recent  able  investi- 
gation of  the  moon's  motion  leads  to  the  value  8.778". 

Another  gravitational  method,  proposed  by  Lev- 
erier,  has  the  advantage  of  cumulative  increase  of 
accuracy.  It  depends  on  the  secular  perturbations 
of  the  orbits  of  the  planets,  especially  of  Venus  and 
the  minor  planets,  by  the  earth,  which  cause  motions 
of  their  nodes  and  periphelia.  As  time  goes  on,  the 
displacements  thus  caused  are  continually  additive, 
and  will  eventually  be  so  large  as  to  be  determined 
with  very  high  accuracy.  Leverier,  indeed,  thought 
it  not  worth  while  to  observe  the  solar  parallax  by 
other  methods,  since  this  must  eventually  lead  them 
all.  Newcomb  gives,  as  the  most  probable  mean  re- 
sult obtained  by  Leverier's  method,  8.768".  A  fav- 
orable application  of  Leverier's  method  may  be  made 
in  the  case  of  Eros.  G.  Witt  has  found,  thus,  the 
ratio  of  the  sun's  mass  to  that  of  the  earth  and  moon 
combined  as  328,882  ±982.  From  this  he  computed 
a  solar  parallax  of  8.794" ±0.009".  Great  improve- 
ment in  the  accuracy  of  this  Eros  result  will  come 
after  the  close  opposition  of  1931. 

It  is  noticeable  that  the  mean  of  the  results 
obtained  by  the  various  gravitational  parallax 

1G 


THE   SUN'S   DISTANCE 

methods  falls  below  that  obtained  by  the  purely 
geometrical  methods  used  in  the  minor  planet  cam- 
paigns. Most  astronomers  would  attribute  this  to  the 
lesser  accuracy  of  the  gravitational  methods  at  pres- 
ent. It  is  certain,  however,  that  the  geometrical 
minor  planet  method  tends  to  give  too  high  results, 
owing  to  the  difference  of  atmospheric  refraction  be- 
tween a  minor  planet,  Eros,  for  instance,  and  the 
comparison  stars;  for  the  minor  planets  shine  by 
reflected  sunlight,  and  their  light  cannot  be  as  rich 
relatively  in  the  blue  end  of  the  spectrum  as  sunlight 
itself  is,  because  of  the  smaller  reflecting  power  of 
nearly  all  solid  substances  for  blue  than  for  red  light. 
On  the  other  hand,  the  light  of  most  stars  is  rela- 
tively richer  in  the  blue  end  of  the  spectrum  than  is 
that  of  the  sun.  It  is  probable,  therefore,  that,  in 
the  mean,  the  comparison  stars  for  Eros,  for  instance, 
are  bluer  than  the  sun,  while  Eros  itself  is  redder  than 
the  sun.  Now  the  point  of  the  method  lies  in  deter- 
mining the  apparent  displacement  of  Eros  at  two 
stations  far  apart  on  the  earth's  surface,  or,  still 
better1,  by  morning  and  evening  observations  at  the 
same  station.  In  the  last-mentioned  method,  when 
Eros  is  low  in  the  east  its  altitude  above  the  horizon 
will  be  increased  by  atmospheric  refraction,  but  the 
comparison  stars,  being  bluer,  will  be  raised  more 
than  Eros.  Similarly  in  the  west.  The  effect  is  to 
make  the  parallax  of  Eros,  and  hence  that  of  the  sun, 
too  large,  no  matter  whether  the  observations  are 
made  simultaneously  by  two  distant  observatories, 

17 


THE  SUN 

or  by  one  observatory  in  the  morning  and  evening. 
It  is  not  yet  determined  how  considerable  this  error 
is,  but  it  should  be  investigated  with  care. 

(3)  Dependence  of  the  Sun's  Distance  on  Geodesy. 

In  all  the  methods  thus  far  referred  to,  the  solar 
parallax  is  obtained  before  the  sun's  distance,  and  the 
astronomer  requires  of  the  geodetic  surveyor  to  tell 
him  the  dimensions  of  the  earth,  if  he  wishes  to  pass 
from  the  parallax  to  the  actual  solar  distance.  Accu- 
rate measurements  of  the  earth  require  pendulum 
observations  at  many  stations  to  fix  the  earth's  shape, 
and,  besides  this,  they  require  the  actual  measure- 
ments by  triangulation  of  very  long  arcs  of  the  earth's 
surface,  and  these  depend  finally  on  measurements 
of  a  base  line  of  a  few  miles  in  length.  Base  lines  are 
measured  by  repeatedly  setting  end  to  end,  under 
microscopic  observation  and  on  specially  leveled 
supports,  short  bars  or  tapes,  whose  temperature 
must  be  observed  throughout  the  process.  Such 
measurements  of  base  lines  are  now  made  with  an 
error  of  less  than  one  part  in  a  million.  The  final 
result  from  the  whole  net  of  triangulation  recently 
completed  across  the  United  States  by  the  Coast  and 
Geodetic  Survey  is  thought  to  be  accurate  to  within 
eighty-five  feet  in  a  total  distance  of  nearly  3,000 
miles.  Several  determinations  of  the  earth's  dimen- 
sions have  been  made  within  fifty  years.  They  give 
the  earth's  mean  equatorial  radius  as  3963.1  miles, 
with  a  probable  error  less  than  one  part  in  20,000. 

18 


THE  SUN'S  DISTANCE 

(4)  Velocity  of  Light  Method. 

We  now  come  to  an  important  class  of  observations 
depending  on  the  velocity  of  light,  and  called  the 
"physical  parallax  methods/'  by  which  we  may  find 
the  sun's  distance  directly.  Several  ways  have  been 
employed  for  measuring  the  velocity  of  light,  but  the 
two  best  are  the  toothed-wheel  method  of  Fizeau  and 
the  revolving  mirror  method  of  Foucault.  In  Fiz- 
eau's  method  (see  Fig.  3)  a  beam  of  light  starts  from 
a  source  at  L  and,  after  passing  through  the  lens  A 
and  being  reflected  by  the  thinly  silvered  plane  glass 


D 


FIG.  3. 

plate  B,  comes  to  focus  and  passes  between  two  teeth 
of  a  wheel  at  F.  Thence  the  ray  goes  on  to  the  lens  C 
and,  after  traveling  a  great  distance,  is  focused  by 
the  lens  D  upon  the  mirror  E,  which  returns  it  on  its 
course,  so  that  at  length  it  passes  again  between  two 
teeth  at  F,  and  a  part  of  it  comes  to  the  observer  at  H. 
Now  imagine  the  wheel  in  rapid  rotation.  The  light 
will  then  be  cut  off  by  every  tooth  which  passes  F, 
and  will  thus  consist  of  a  series  of  flashes.  But, 
owing  to  the  persistence  of  vision,  the  beam  will  still 
seem  continuous  if  observed  at  H,  though  it  will 
be  weakened  because  it  shines  only  intermittently. 

19 


THE  SUN 

Time  is  required,  however,  for  the  light  to  pass  from 
F  to  E  and  back  to  F,  and  meanwhile  the  tooth  next 
F  has  advanced,  and  may  be  in  such  a  position  as 
exactly  to  cut  off  the  returning  beam,  so  that  the  eve 
at  H  will  see  nothing.  By  gradually  increasing  the 
speed  of  the  wheel  the  light  is  alternately  cut  off  and 
transmitted.  By  counting  these  changes  from  light 
to  darkness,  and  knowing  the  number  of  teeth  and  the 
speed  of  the  wheel . and  the  distance  FE,  the  velocity 
of  light  is  measured. 

The  method  of  Foucault  is  illustrated  by  Fig.  4. 
Light  from  the  slit  S  passes  through  the  thinly  sil- 

B A P 


0 


vered  glass  plate  P,  thence  through  the  lens  A,  and  is 
reflected  by  a  plane  mirror  B  to  the  concave  mirror  C. 
The  radius  of  curvature  of  the  mirror  C  is  equal  to 
BC,  so  that  the  light  is  returned  to  B  in  the  same  path 
that  it  traversed  in  going,  and  thus  it  again  passes 
through  the  lens  A,  and  a  part  is  reflected  by  the  sil- 
vered glass  P  and  is  observed  at  0.  If  the  mirror  B 
is  revolved  slowly,  the  light  comes  out  at  O  as  a 
series  of  flashes.  These  become  sensibly  continu- 
ous to  the  eye  as  the  speed  increases,  but  when  the 
speed  becomes  high  the  image  at  O  becomes  dis- 

20 


THE  SUN'S   DISTANCE 


placed  owing  to  the  motion  of  rotation  of  the  mirror 
B  while  the  light  is  passing  from  B  to  C  and  back  to 
B.  From  the  amount  of  displacement,  the  speed  of 
the  mirror,  and  the  distance  BC,  the  velocity  of  light 
is  computed. 

According  to  the  electromagnetic  theory  of  light, 
the  ratio  between  the  electrostatic  and  the  electro- 
magnetic systems  of  electrical  units  should  also  give 
the  velocity  of  light.  Furthermore,  the  electric  waves 
used  in  wireless  telegraphy  should  proceed  with 
the  velocity  of  light.  Results  depending  on  these 
last  two  considerations,  though  interesting,  cannot 
rival  in  accuracy  the  velocity  directly  determined. 
The  following  are  among  the  best  results : 


OBSERVER. 

Method. 

VELOCITY  OF  LIGHT  m 

VACUO 

Kilometers 
per  second. 

Miles 
per  second. 

Mean  of  Michelson,  New-  ) 
comb  and  others  f 
Mean  of  Cornu  and  Per-  \ 
rotin                                  f 

Foucault  

Fizeau  

Hertz  waves  .  . 
Ratio  of  units  . 

299,860 

299,890 
299,130 
299,710 

186,330 
186,350 

Various  observers  
Rosa  and  Dorsey  

Accepted  velocity  of  light  .  . 



299,860 

186,330 

The  velocity  of  light  just  given  is  probably  correct 
within  one  part  in  ten  thousand. 

There  are  three  ways  of  employing  this  quantity 
to  fix  the  distance  of  the  sun.  The  first  we  will  men- 
tion is  through  the  aberration  of  light.  Though  light 
proceeds  by  wave  motions,  and  not  by  particles,  the 

21 


THE  SUN 


idea  of  aberration  may  be  understood  by  the  analogy 
of  raindrops.  If  rain  is  falling  vertically,  and  a  man 
stands  still,  his  hat  screens  his  face.  But  if  he  move 
rapidly  forward  in  any  direction,  the  rain  strikes  his 
face,  thus  appearing  not  to  come  vertically  but  at  an 
angle  thereto.  So  with  the  light  of  the  stars:  Owing 
to  the  motion  of  the  earth  in  its  orbit  the  stars  are 
apparently  displaced  when  the  earth  is  moving  at 
right  angles  to  the  line  of  sight;  the  displacement 
being  in  one  direction  at  one  time  of  the  year  and  in 
the  opposite  direction  when  the  earth's  motion  is  re- 
versed six  months- afterwards.  Owing  to  aberration, 
stars  at  the  poles  of  the  ecliptic  describe  little  circles 
about  41"  in  diameter,  and  those  in  the  plane  of  the 
ecliptic  merely  oscillate  in  a  straight  line  about  41 " 
long.  There  is  an  uncertainty  of  a  few  hundredths 
of  a  second  as  to  the  " constant  of  aberration,"  as 
astronomers  call  the  radius  of  the  circle  of  aberration. 
The  Paris  conference  of  1896  adopted  the  value 
20.47".  But  there  is  now  much  evidence  tending  to 
recommend  a  higher  value.  The  long-continued  ob- 
servations of  Doolittle  made  with  instruments  of 
different  kinds  seem  to  require  us  to  set  the  constant 
of  aberration  as  high  as  20.51",  perhaps  even  20.53". 
The  following  table  shows  the  relation  of  aberration 
and  solar  parallax  values : 


Aberration 
Constant. 

20.46" 

20.47" 

20.48" 

20.49" 

20.50" 

20.51" 

20.52" 

20.53" 

Solar.  .:*... 
Parallax. 

8.807" 

8.803" 

8.799" 

8.794" 

8.790" 

8.786" 

8.781" 

8.777" 

22 


THE  SUN'S  DISTANCE 

Another  way  to  use  the  velocity  of  light  for  fixing 
the  sun's  distance  is  through  the  observations  of 
Jupiter's  satellites.  Olaf  Homer  called  attention  to 
this  method  by  taking  the  problem  the  other  way 
about  and  computing  the  velocity  of  light  from  the 
supposed  known  distance  of  the  sun.  The  satellites 
pass  behind  the  planet  and  are  eclipsed  frequently. 
These  eclipses  occur  nearly  1,000  seconds  later  in 
time  when  Jupiter  is  in  conjunction  than  when  in  op- 
position, for  there  is  a  difference  of  distance  amount- 
ing to  the  whole  diameter  of  the  earth's  orbit  for 
light  to  traverse  in  the  two  cases.  Unfortunately,  the 
eclipses  are  not  sudden  phenomena,  so  that  it  requires 
careful  photometric  work  to  fix  the  "light  equation," 
as  the  time  required  for  light  to  travel  the  radius 
of  the  earth's  orbit  is  called.  According  to  many 
years  of  observation  at  the  Harvard  College  Obser- 
vatory, as  reduced  by  Professor  Samson  of  Durham, 
the  light  equation  is  498.64  seconds.  From  this  the 
sun's  parallax  comes  out  S.799". 

A  third  way  of  employing  the  velocity  of  light 
is  through  what  is  known  as  the  Doppler  effect. 
Just  as  a  locomotive  whistle  is  higher  in  pitch  when 
the  train  approaches,  so  the  light  of  a  star  is  bluer 
in  hue  when  we  are  approaching  the  star  in  the  earth's 
orbit.  The  velocity  of  the  earth  is  so  small  compared 
to  the  velocity  of  light  that  the  magnitude  of  the 
change  can  be  measured  only  with  a  powerful  spec- 
troscope of  special  design.  Nevertheless,  it  seems 
possible  that  solar  parallax  determinations  by  this 
4  23 


THE  SUN 

method  may  before  long  compare  favorably  in 
accuracy  with  any  others.  For  parallax  purposes, 
Kiistner,  and  lately  Halm,  have  photographed  the 
spectra  of  bright  stars  in  comparison  with  spark 
spectra  photographed  above  and  below  on  the  same 
plate.  They  repeated  these  comparisons  at  inter- 
vals of  about  six  months  for  several  years,  and,  after 
applying  necessary  corrections,  they  have  determined 
the  velocity  of  the  earth  in  its  orbit,  and  from  this 
the  sun's  parallax.  Their  values  are  not  far  from 
those  obtained  from  other  methods,  but  are  not 
quite  accurate  enough  to  compete  with  them. 

It  would  seem  more  promising  to  determine  the 
relative  velocities  of  the  planets  and  the  earth,  by 
photographing  simultaneously  the  spectra  of  Venus 
and  Mars,  or  of  Mars  and  the  moon  at  a  favorable 
time.  Suppose,  for  instance,  that  two  large  coe- 
lostats  (see  Chapter  II)  were  arranged  one  beside  the 
other  to  reflect  the  light  of  Mars  and  Venus  simul- 
taneously upon  a  single  long-focus  concave  mirror, 
and  the  two  images  were  reflected  together  by  suit- 
able devices  so  as  to  fall  at  once  one  above  the  other 
on  the  slit  of  a  powerful  spectroscope.  By  the  use 
of  a  rotating  sector  one  image  could  be  made  equal 
to  the  other  in  brightness,  and  the  spectra  of  both 
could  then  be  photographed  absolutely  simultane- 
ously, one  above  the  other  on  the  same  plate. 
Errors  such  as  displacements  by  change  of  tempera- 
ture of  the  spectroscope  would  effect  both  spectra 
alike.  The  coelostats  should  be  used  alternately 

24 


THE  SUN'S  DISTANCE 

on  the  two  objects,  so  as  to  follow  out  Professor 
Turner's  excellent  motto  of  "reversing  everything 
that  can  be  reversed."  From  preliminary  trials 
made  at  Mount  Wilson  it  seemed  to  Mr.  Adams  and 
the  writer  that  it  would  be  practicable  to  photo- 
graph the  two  spectra  on  a  scale  -5-  as  large  as  that 
which  Mr.  Adams  employed  to  determine  the  solar 
rotation  spectroscopically.  As  the  spectra  of  the 
planets  are  similar,  being  solar  spectra  slightly 
altered  by  selective  reflection,  there  would  be 
numerous  good  lines  to  measure.  There  would 
evidently  be  no  necessity  of  introducing  a  terres- 
trial comparison  spectrum.  It  seems  probable  that 
by  this  method  the  solar  parallax  could  be  determi- 
nable  to  about  one  part  in  2000.  However,  it  has 
not  yet  been  tried. 

(5)  Summary. 

Excluding    parallax    values    not    of    the    highest 
weight,  we  have  the  following  mean  results: 

From  heliometer  work  on  minor  planets  . 8 . 807" 

From  the  Eros  campaign 8 . 807" 

From  all  gravitational  methods 8 . 780" 

From  eclipses  of  Jupiter's  satellites1 : 8 . 799" 

From  the  constant  of  aberration  of  light  (assumed  20 . 51"). . .  8 . 786" 

If  we  take  the  mean  of  all  these  results  as  they  stand, 
we,  in  effect,  give  double  weight  to  the  geometrical 
and  velocity  of  light  methods  as  compared  with  the 

1  This  determination  is  included,  notwithstanding  the  large  probable  error 
Professor  Samson  assigns  to  it,  because  in  the  author's  opinon,  as  indicated  above, 
the  constant  of  aberration  and  the  minor  planet  parallaxes  are  uncertain  to  nearly 
the  same  degree,  and  the  value  of  an  independent  method  is  very  great. 

9'K 


THE  SUN 

gravitational  method.  This  seems  justified,  and 
by  doing  so  we  reach  the  probable  value  of  the 
solar  parallax  as 

8.796". 

This  corresponds  to  a  solar  distance  of 

92,930,000  miles  or  149,560,000  kilometers. 

DIAMETER  OF  THE  SUN 

From  the  heliometer  measurements  of  Schurr  and 
Ambronn  the  sun's  angular  diameter,  as  seen  from 
the  earth  at  mean  distance,  is  1920.0"  ±0.03."  Other 
determinations  agree  very  closely  with  this.  Hence, 
the  sun's  diameter  is 

865,000  miles  or  1,392,000  kilometers. 
Poor  has  lately  maintained  that  observations  indi- 
cate that  the  sun's  equatorial  and  polar  diameters 
vary  relatively  as  much  as  0.1"  during  a  sun-spot 
cycle  of  eleven  years.  According  to  him,  the 
equatorial  diameter  is  the  larger  at  sun-spot  maxi- 
mum, and  the  polar  diameter  the  larger  at  minimum. 
Ambronn,  however,  denies  that  this  is  supported 
by  the  observations,  and  Moulton  opposes  so  large  a 
variation  on  theoretical  grounds. 

THE  SUN'S  MASS 

The  mass  of  the  sun  relatively  to  a  planet  which  has 
a  satellite  may  be  obtained  in  several  ways.  One 
of  them,  as  applied  to  the  earth,  is  as  follows: 
Let  M  be  the  mass  of  the  sun,  earth  and  moon 

26 


THE  SUN'S   DIMENSIONS 

combined;  and  m,  that  of  the  earth  and  moon;  let 
R  be  the  mean  distance  between  the  centers  of 
the  sun  and  the  earth;  r,  the  mean  distance  between 
centers  of  the  earth  and  moon;  let  T  be  the  number 
of  days  in  a  sidereal  year,  and  t  the  number  in  a 
sidereal  month.  Then,  by  Kepler's  law: 

R3     r3  R3     r3     r3 

M:m  =  —  :-;  whence  M-ra  :ra  =———:—. 

The  mass  of  the  moon  compared  with  the  earth  is 
known  from  other  data  to  be  1/81.53.  Small  cor- 
rections to  the  periods  T  and  t  due  to  the  perturba- 
tions are  also  known.  Applying  these  corrections, 
the  ratio  of  the  masses  of  the  sun  and  earth  comes 
out.  It  is,  according  to  Newcomb,  for  a  parallax 
of  8.796": 

332,800. 

THE  EARTH'S  MASS 

Astronomy  must  be  displaced  by  physics  if  we 
would  proceed  further  and  obtain  the  mass  of  the  sun 
in  ordinary  units;  for  the  mass  of  the  earth  is  then 
required.  This  is  determined  by  comparing  the  at- 
traction of  the  earth  for  a  body  (i.e.,  the  weight  of  the 
body)  with  the  attraction  which  a  known  mass  pro- 
duces when  acting  upon  the  body  from  a  known  dis- 
tance. During  the  eighteenth  century  attempts  were 
made  to  compare  the  attraction  of  a  mountain  with 
that  of  the  earth.  The  most  celebrated  of  these  was 
performed  in  1775,  under  the  auspices  of  the  Royal 
Society,  by  the  Astronomer  Royal  Maskelyne  at-  the 

27 


THE  SUN 

mountain  Schehallien  in  Scotland.  Owing  to  the  im- 
possibility of  determining  accurately  the  center  of 
gravity  and  density  of  a  mountain,  this  method, 
though  suggested  by  Newton,  and  one  very  interest- 
ing to  contemplate,  is  of  little  value.  What  is  known 
as  the  method  of  Cavendish,  though  proposed  by 
Michell,  is  regarded  as  best.  In  this  method  a  pair 
of  small  balls  are  hung  at  the  ends  of  a  rod  which  is 
supported  in  the  center  by  a  fine  wire  or  fiber.  Two 
large  masses  are  placed  in  a  position  to  twist  the  sus- 
pending fiber  by  attracting  the  small  balls.  The 
force  of  attraction  is  measured  by  the  torsion  of  the 
fiber,  and  this  is  determined  by  the  period  of  vibra- 
tion of  the  system.  C.  V.  Boys  performed  a  notable 
piece  of  work  by  this  method  in  1894,  and  for  it  he 
invented  the  quartz  fiber,  without  which  some  of  the 
most  delicate  and  interesting  of  modern  physical 
work  in  other  lines  would  be  impossible.  His  first 
method  of  making  quartz  fibers  was  very  picturesque. 
One  bit  of  a  quartz  crystal  being  fastened  to  a  little 
arrow,  and  another  to  a  bow,  the  two  bits  are  fused 
together  by  a  blowpipe  flame;  and  when  the  quartz 
is  properly  melted  the  arrow  is  shot  out,  trailing  be- 
hind it  a  thread  of  quartz  almost  too  fine  to  be  seen. 
Such  fibers  are  almost  perfectly  elastic,  and  as  strong 
as  steel  in  proportion  to  their  size.  Boys  obtained 
for  the  mean  density  of  the  earth,  5.527.  In  other 
words,  the  earth  has  five  and  a  half  times  as  great 
mass  as  it  would  have  if  composed  entirely  of  water. 
The  principle  of  a  third  method,  more  simple  to  un- 

28 


THE  SUN'S   DIMENSIONS 


BO 


FIG.  5. 


derstand  than  Cavendish's,  is  illustrated  in  Fig.  5. 
Two  equal  balls,  A  and  B,  are  suspended  from  the 
beam  E  of  an  equal  arm  balance,  and  two  large  equal 
balls  are  arranged  so  as  to  be  in  the  positions  C  and 
D,  or  C'  and  D',  at  pleasure.  In  the  first  position 
they  tend  by  their  at- 
tractions to  make  A 
overbalance  B,  and  in 
the  second  position,  the 
opposite.  Hence  the 
mere  weighing  of  B 
against  A  by  means  of  (  D' 
a  rider  on  the  beam  E  is  NV-- 
the  principal  require- 
ment in  addition  to  knowing  the  masses  of  the 
balls,  C  and  D,  and  their  distances  from  A  and 
B.  By  this  means  Richarz  and  Krigar-Menzel 
determined  the  earth's  mean  density  to  be  5.505. 
Burgess  has  discussed  the  different  determina- 
tions, and  gives  the  most  probable  value  of  the 
earth's  density  as  5.5247  ±  0.0013.  Corresponding 
to  this,  the  constant  of  gravitation  (see  the  begin- 

— 3 
cm 
ning  of  this  chapter)  is  666.07  X  10~10-      ^r   (or 

gr.  sec.2 

dynes).  From  these  figures  the  mass  of  the  earth  is 
1.317  X  1025  pounds,  or  5.984  x  1024  kilograms,  and 
that  of  the  sun  is  4.38  X  1030  pounds,  or  1.990  X 
1030  kilograms  (that  is  4,  or  1,  followed  by  30  places 
of  zeros). 

29 


THE  SUN 

• 

THE  SUN'S  DENSITY 

Since  the  volume  of  the  sun  is  1,306,000  times  that 
of  the  earth,  the  density  of  the  sun  is  only  0.255  as 
great  as  that  of  the  earth,  and  is  1.41  as  compared 
with  water.  A  most  interesting  and  important  con- 
clusion follows  from  these  figures  on  the  density  of 
the  sun.  Notwithstanding  that  its  density  is  so 
small,  we  know  from  its  spectrum  that  the  sun  has 
many  of  the  heavy  metals  and  other  chemical  ele- 
ments found  upon  the  earth,  and  we  presume  that  it 
includes  few  elements  or  compounds  which  in  a  liquid 
or  solid  state  would  be  of  less  density  than  water. 
Water  and  other  common  liquids  can  not  exist  even 
as  vapors  on  the  sun,  owing  to  the  high  temperature. 
In  view  of  these  facts,  it  follows  that  the  sun  is  prob- 
ably mainly  gaseous.  Owing  to  the  enormous  mass  of 
the  sun,  the  attraction  of  gravitation  at  its  surface  is 
27.6  times  as  great  as  that  at  the  surface  of  the  earth, 
so  that  a  body  which  weighed  a  hundred  pounds  here 
would  weigh  over  a  ton  there.  Hence,  the  gases 
of  the  interior  of  the  sun  must  be  tremendously  com- 
pressed, so  that  probably  in  their  appearance  they 
would  resemble  liquids,  though  still  having  the  prop- 
erty of  indefinite  expansibility  characteristic  of  gases. 


CHAPTER   II 

THE     INSTRUMENTS     AND     METHODS     USED     IN  SOLAR 
INVESTIGATION 

The  Telescope.— The  Coclostat.  —  The  Spectrum  and  what  it  In- 
dicates.— •  Spectroscopes. — The  Spectroheliograph. — The  Helio- 
micrometer. — The  Comparator. — The  Nature  of  Radiation. — 
Laws  of  Radiation. — Spectra  of  Different  Sources. — Pyrheliom- 
etry. — Bolometry. 

FOR  a  long  time  the  telescope  and  the  observer's 
eye  were  the  principal  means  of  advancing  solar  in- 
vestigation, but  in  the  last  half  century  a  number  of 
other  less  familiar  instruments  and  physical  princi- 
ples have  been  employed,  which  require  some  ex- 
planation. 

THE  TELESCOPE 

A  few  words  may  be  said  first  as  to  the  methods  of 
employing  the  telescope.  The  sun  is  far  too  bright 
to  view  for  any  length  of  time  with  the  naked  eye, 
much  less  with  the  telescope,  unless  means  are  used 
for  reducing  the  brightness.  It  is  said  that  the  Bel- 
gian physicist,  Plateau,  having  looked  steadily  at  the 
sun  twenty  seconds  for  the  purpose  of  studying  the 
after  images  which  would  be  produced,  lost  his  sight 
permanently  in  consequence. 

To  get  a  rough  general  view  of  the  sun  a  screen  is 
often  used  in  the  manner  shown  in  Fig.  6.  The  dis- 

31 


THE  SUN 

tance  of  the  screen  from  the  eyepiece  depends  on 
the  size  of  the  image  desired  and  the  power  of  the 
eyepiece.  By  moving  the  eyepiece  to  and  fro  in 
the  draw  tube,  a  sharp  image  is  readily  obtained. 
It  is  well  to  put  a  screen  on  the  front  of  the  tele- 
scope, as  shown  in  the  figure,  to  cut  off  undesired 
light.  For  Carrington's  method  of  determining  the 
exact  locations  of  sun  spots  on  the  disk,  see  Monthly 


FIG.  6. 

Notices  of  the  Royal  Astronomical  Society,  vol.  xiv, 
p.  153. 

Observations  of  the  finer  details  of  the  sun  cannot 
be  made  with  a  screen,  and  there  are  several  ways 
of  protecting  the  eye  for  direct  telescopic  vision. 
This  may  be  done  by  reducing  the  aperture  of  the 
telescope  objective  with  a  suitable  diaphragm  and 
placing  a  dark  glass  in  front  of  the  eyepiece,  but  at 

33 


SOLAR  INVESTIGATION 


great  cost  of  definition  if  the  diaphragm  is  too  small 
or  the  shade  glass  not  perfect.  A  reflecting  tele- 
scope, if  its  object  mirror  is  left  unsilvered,  may  re- 
quire only  a  shade  glass  for  visual  work  with  the  sun; 
and,  on  the  other  hand,  the  objective  lens  of  a  re- 
fractor may  be  thinly  silvered  to  cut  down  the  light. 
But  both  these  expedients  unfit  the  telescope  for 
other  purposes.  There  are  several  special  solar 
eyepieces  which  have 
been  devised.  Fig.  7 
shows  Sir  John  Her- 
schel's.  The  light  en- 
tering at  O  encounters  a 
prism  of  glass  whose  first 
surface  is  placed  at  an 
angle  of  45°.  More  than 
90  per  cent  of  the  light 
passes  through  the  prism 
and  goes  out  through 
the  open  end  of  the  tube,  while  the  reflected  light 
goes  up  through  the  eyepiece  AB.  A  shade  glass  is 
still  necessary,  but  need  not  be  very  dark.  It  is 
advantageous  to  employ  a  long  thin  wedge  of  dark 
glass  (" London  Smoke,"  for  instance)  compensated 
by  a  corresponding  wedge  of  ordinary  glass,  as 
shown  in  Fig.  8. 

With  this  arrangement  the  image  is  undistorted, 
uncolored,  and  may  be  made  of  exactly  the  proper 
brightness  for  observing.  The  polarizing  eyepieces 
on  the  general  plan  shown  in  Fig.  9  are  more  con- 

33 


FIG. 


THE  SUN 


FIG. 


venient,  but  also  more  expensive.     The  light  is  re- 
duced  merely   by  rotating   the   upper   case  in   its 

{^^•^•^^^•^•^^•••1     bearing     in     the 

lower.  The  image 
is  seen  in  its 
proper  color  and 
without  being 
either  inverted 
or  reversed  from 
right  to  left 
by  the  eyepiece. 

When  very  small  objects  are  being  examined,  it  is 

sometimes  advantageous  to  use  Dawes's  device  of 

limiting  the   field  of   view    by  means  of  a  minute 

diaphragm    made    by 

piercing  a  card  or  plate 

of   ivory  with    a   hot 

needle. 

For       photographic 

work      the      extreme 

brightness  of  the  sun 

is    advantageous     in- 
stead, of  troublesome, 

because  it  enables  the 

observer     to     employ 

slow  plates  which  have 

much  finer  grain  than 

rapid  ones,  and  also  to 

cut  down  the  exposure 

time,  which  is  an  im-  FlG  9 


SOLAR  INVESTIGATION 


portant  gain,  since  it  is  favorable  to  making  com- 

plete exposures  during    the  occasional   instants   of 

exceptionally    good    atmospheric    conditions   which 

favor  superior  optical  definition  or  "good  seeing." 

As  all  solar  observers  know,  the  atmospheric  effect 

called  "  boiling"  is  generally  much  worse  in  the  day- 

time than  in  the  night,  and  undoubtedly  because 

the  powerful  heating  of  the  surface  of  the  ground 

by  the  solar   rays 

causes    rising    air 

currents      of     un- 

equal   density, 

which  drift  hither 

and  thither  across 

the  line   of   sight. 

Photographs  of  the 

sun  are  usually  ex- 

posed by  means  of 

a    sliding    shutter 

similar  in  action  to 

the  form  shown  in  Fig.  10.      B  is  a  catch  which 

may  be  released  by  the  electromagnet,  or  by  hand, 

thus  allowing  the  spring  S  to  draw  the  slide  con- 

taining the   slit  A   swiftly  across   the   opening  O, 

through  which  the  rays  enter  the  camera.     The  ex- 

posure is    proportional    to    the  width    of    the  slit 

A,  and  is  governed  by  the  tension  of  the  spring  S. 


FIG.  10. 


to  —  —  of  a  second  are  required 


Exposures  of 


according  to  circumstances.     The  edges  of  the  sun's 

35 


THE  SUN 

disk  are  not  as  bright  as  the  center,  so  that  the  solar 
image  cannot  be  properly  exposed  to  show  details 
equally  well  in  all  parts  in  the  same  picture. 

It  is  often  desirable  to  know  the  orientation  of  the 
solar  image.  For  this  purpose  a  cord  or  wire  may 
be  stretched  close  to  the  plate,  in  some  known 
position,  as  horizontal,  or  parallel  to  the  sun's 
drift,  or  vertical,  and  its  shadow  on  the  image  will 
serve  as  a  basis  of  computation.  Sometimes  in  an 
optical  system  containing  reflectors  it  is  desired 
to  know  what  parts  of  the  solar  image  correspond  to 
east  and  west  in  the  sky.  This  may  always  be 
determined  with  ease  and  certainty  by  stopping  the 
telescope  motion  and  letting  the  sun's  image  drift; 
for  the  advancing  edge  or  "limb"  of  the  image  must 
always  correspond  to  the  west  edge  or  "limb"  of  the 
sun  in  the  sky.  The  data  for  computing  the  position 
of  the  sun's  equator  are  published  annually  in  the 
pamphlet  called  The  Companion  to  the  Observa- 
tory. 

THE  CCELOSTAT 

Most  modern  apparatus  for  solar  research  is  of  a 
complex  and  necessarily  bulky  nature,  so  that  it  is 
highly  inconvenient  to  move  it.  Accordingly,  a  fixed 
beam  of  sunlight  is  almost  a  necessity.  There  are 
several  kinds  of  instruments  for  reflecting  the  light  of 
heavenly  bodies  in  a  fixed  direction,  called  heliostats 
or  siderostats;  but  these  all  rotate  the  image  of  the 
heavenly  object  if  an  image  is  formed.  This  is  usu- 
ally a  great  disadvantage,  and,  fortunately,  there  is 

36 


PLATE  I 


SMITHSONIAN  OBSERVING  SHELTER  AND  CCELOSTAT,  MT.  WILSON. 


SOLAR  INVESTIGATION 

one  very  simple  instrument  for  the  purpose,  called  the 
ccelostat,  which  does  not  rotate  the  image.  In  its 
simplest  form  the  ccelostat  is  a  single  plane  mirror 
mounted  on  an  axis  parallel  to  the  axis  of  the  earth, 
and  rotated  by  clockwork  at  the  rate  of  one  complete 
rotation  in  forty-eight  hours.  In  this  form  the  sun's 
beam  is  reflected  in  a  different  direction  at  different 
times  of  the  year,  according  to  the  position  of  the  sun 
north  or  south  of  the  celestial  equator.  Even  in  a 
single  day  the  mirror  cannot  be  used  to  throw  a  hori- 
zontal beam  in  any  desired  direction,  but  only  in  two, 
nearly  east  and  west  respectively,  the  first  favorable 
for  morning  hours,  the  other  for  afternoon  hours. 
These  limitations  are  overcome  by  the  introduction 
of  a  second  plane  mirror,  south  of  and  above  the  level 
of  the  first,  on  which  the  beam  is  first  reflected,  and 
from  which  it  can  be  sent  in  any  desired  direction, 
but  preferably  (for  the  northern  hemisphere)  towards 
the  north.  It  is  necessary  to  provide  longitudinal 
and  cross  motions  for  one  or  the  other  of  the  mirrors, 
to  accommodate  the  change  in  declination  of  the  sun 
at  different  times  of  the  year.  Plate  I  shows  the 
fifteen-inch  ccelostat  of  the  Smithsonian  Astrophysi- 
cal  Observatory  at  Mount  Wilson,  Qal.  The  first 
or  rotating  mirror  is  provided  with  means  of  moving 
it  on  tracks  both  east  and  west,  and  also  north  and 
south.  The  second  or  fixed  mirror  reflects  the  beam 
horizontally  northward  to  the  spectroscope  within 
the  observatory. 


37 


THE  SUN 

THE  SPECTRUM  AND  WHAT  IT  INDICATES 

Since  Kirchhoff  and  Bunsen's  great  discovery  of 
spectrum  analysis  in  1859,  the  spectroscope  has  be- 
come more  and  more  indispensable  to  progress  in 
solar  research,  so  that  now  the  greater  portion  of  our 
knowledge  of  the  sun  is  due  to  this  instrument.  White 
light  is  not  a  simple  but  a  composite  thing,  containing 
potentially  all  the  different  colors  familiar  to  the  eye, 
and  still  other  rays  which  the  eye  sees  not  at  all.  As 
we  shall  describe,  light  can  be  analyzed  so  as  to  pre- 
sent to  the  eye  the  colors,  and  this  presentation  is 
usually  in  the  form  of  a  long  ribbon  of  color  gradation. 
When  light  is  thus  analyzed  to  show  the  colors  which 
are  potentially  in  it,  the  spectrum  is  said  to  be  pro- 
duced. 

If  sunlight  is  resolved  into  a  spectrum,  under  good 
conditions  we  see  a  ribbon  of  light  shading  gradually 
from  dull  red  through  brighter  and  brighter  hues  to 
orange,  then  yellow,  next  green,  then  blue,  indigo  and 
violet.  If  we  had  eyes  of  unlimited  capacity  we 
should  see  beyond  the  violet  still  other  rays,  and 
beyond  the  red  yet  others,  also.  We  can  detect 
such  invisible  rays  by  the  heat  they  produce  or  by 
photography,  but  just  as  the  ear  cannot  hear  sounds 
above  a  certain  pitch,  or  below  a  certain  other  pitch, 
the  eye  is  limited  as  to  its  recognition  of  radiation. 
Rays  lying  beyond  the  violet  end  of  the  visible  spec- 
trum are  called  "  ultra-violet "  and  those  beyond 
the  red  are  called  "  infra-red.'7  In  the  visible  spec- 

38 


SOLAR  INVESTIGATION 

trum  the  shading  is  not  perfectly  continuous,  for 
there  may  be  seen  almost  innumerable  vacancies  of 
color,  or  dark  lines  crossing  the  colored  ribbon  at 
right  angles.  These  dark  lines  are  called,  after  the 
name  of  their  discoverer,  Fraunhofer  lines.  It  is 
their  presence,  and  not  the  beautiful  colors,  which 
has  been  the  means  of  teaching  us  many  things  about 
the  sun  and  stars  which  would  have  seemed  to  the 
contemporaries  of  the  Herschels  to  be  beyond  the 
possibilities  of  future  discovery. 

The  cause  of  the  dark  lines  of  the  spectrum  was 
unknown  until  Kirchhoff  and  Bunsen's  researches, 
about  1859,  showed  that  they  correspond  in  position 
to  certain  bright  lines  which  form  the  spectra  of  me- 
tallic vapors.  For  example,  if  metallic  sodium,  or 
any  of  its  compounds  like  common  table  salt,  is 
thrust  into  an  alcohol-lamp  flame,  the  spectrum  of 
the  flame  shows  two  brilliant  yellow  lines  which  agree 
in  place  with  two  prominent  dark  lines  in  the  yellow 
part  of  the  solar  spectrum.  Not  only  so,  but  if  an 
incandescent  oxy-hydrogen  calcium  light,  whose  nat- 
ural spectrum  shows  neither  bright  nor  dark  lines  in 
the  yellow  at  these  places,  is  caused  to  shine  through 
an  alcohol-lamp  flame  charged  with  sodium  vapor  and 
placed  before  the  spectroscope,  the  two  dark  lines 
like  those  in  the  solar  spectrum  will  appear  instead  of 
the  two  bright  lines  of  the  sodium-charged  flame  it- 
self. Other  chemical  elements,  also,  when  heated  to 
vaporization,  emit  bright  spectrum  lines,  and  the 
vapors  of  these  elements,  if  placed  in  a  beam  of  white 
5  39 


THE  SUN 

light,  absorb  the  rays  they  themselves  emit.  If  their 
own  emission  is  more  intense  than  the  emission  they 
absorb  from  such  a  transmitted  beam,  the  effect  will 
be  brighter  lines  in  a  continuously  bright  spectrum. 
If  their  own  emission  is  less  intense  than  the  emission 
they  absorb,  the  resulting  spectrum  will  be  crossed 
by  dark  lines.  The  former  effect  occurs  in  the  spec- 
tra of  certain  stars,  the  latter  in  that  of  the  sun.  As 
the  emission  of  a  vapor  falls  off  rapidly  as  the  temper- 
ature falls,  it  is  natural  to  suppose  as  the  cause  of  the 
sun's  characteristic  dark  spectrum  lines  that  the 
metallic  vapors  of  the  sun's  outer  layers,  since  they 
are  free  to  loose  heat  to  space,  continue  cooler  than 
the  sun's  inner  layers,  and  hence  cannot,  by  their  own 
emission,  fully  compensate  and  supply  the  place  of 
the  rays  they  absorb. 

In  Fig.  1  of  Plate  IV  a  part  of  the  spectrum  of  the 
star  Procyon  is  shown,  with  comparison  spectra  of 
iron  above  and  below  it.  The  stellar  spectrum  shows 
numerous  dark  lines  for  the  reasons  indicated  above; 
and  many  of  these  correspond  closely  in  position  and 
relative  importance  (or  intensity,  as  it  is  called)  to 
bright  lines  in  the  iron  spectrum.  For  a  reason  to  be 
explained  below,  the  stellar  lines  are  all  shifted  a  little 
towards  the  violet  with  respect  to  the  comparison 
spectrum,  but  it  is  evident  that  iron  has  left  its  sign 
in  the  star's  spectrum  as  well  as  in  that  of  the  electric 
spark. 

First  of  all,  then,  the  dark  lines  of  the  solar  and 
stellar  spectra  show  what  chemical  elements  are  pres- 

40 


SOLAR  INVESTIGATION 

ent  in  the  sun  and  stars.  By  comparing  the  solar 
spectrum  with  bright  line  spectra  of  pure  metals  pro- 
duced in  the  laboratory,  it  has  been  shown  that 
nearly  all  of  the  elements  found  in  the  sun  are  also 
present  in  the  earth.  In  the  second  place  the  lines 
of  the  solar  spectrum  serve  as  reference  marks  to  en- 
able us  to  recognize  the  effect  of  certain  influences  in 
the  sun,  such  as  varying  degrees  of  temperature,  of 
velocity,  of  pressure,  and  of  magnetism. 

As  regards  temperature:  A  dark  spectrum  line 
generally  indicates  a  cooler  vapor  in  front  of  a  hotter 
source,  and  a  bright  line  that  no  hotter  source  lies 
behind.  Furthermore,  many  elements  give  in  the 
laboratory  a  large  number  of  bright  lines  whose 
relative  intensities  differ  according  to  the  tempera- 
ture of  the  source.  Similar  differences  of  intensity 
among  the  lines  of  a  given  element,  as  found  in  the 
solar  spectrum,  give  a  basis  for  estimating  differ- 
ences of  temperature  there,  as,  for  instance,  between 
a  sun  spot  and  the  photosphere. 

As  regards  velocity:  We  have  noted  the  Dop- 
pler  effect  already  in  speaking  of  methods  of  meas- 
uring the  sun's  distance.  It  depends  on  the  fact 
that  light  travels  by  waves.  Those  waves  which 
are  visible  to  the  eye,  differ  in  length  from  0.0004 
millimeter  (0.4  p)  to  0.0007  millimeter  (0.7  p),  cor- 
responding to  violet  and  dull  red  respectively.. 
The  time  period  of  a  complete  vibration  of  a  wave  of 
violet  light  is  given  by  the  ratio  of  its  length  (.0004 
millimeter)  to  the  velocity  of  light  (300,000,000,000 

41 


THE  SUN 

millimeters  per  second)  .  Hence,  750,000,000,000,000 
waves  of  violet  light  are  emitted  from  all  parts  of 
the  sun's  surface  each  second.1  Doppler's  principle 
is  as  follows  :  If  a  star  is  approaching  the  earth  with 
a  velocity  v,  the  effect  is  to  shorten  the  length  of  each 
wave  of  light  reaching  the  earth  by  an  amount  vt, 
where  t  is  the  period  of  vibration  of  the  wave.  If 
V  is  the  velocity  of  light  and  X  the  original  wave- 
length, X  =  Vt.  Suppose  the  apparent  wave- 
length to  be  X,,  then  X,  =  (V—  v)t.  Hence,  (X—  \) 


If  the  lines  of  a  spectrum  are  displaced  towards  the 
violet  by  amounts  which  are  less  at  the  violet  end 
of  the  spectrum  than  at  the  red  in  the  ratio  of  the 
wave  lengths,  this  may  be  an  indication  that  the 
source  of  light  is  approaching  the  earth.  By  com- 
paring the  positions  of  the  spectrum  lines  at  different 
parts  of  the  edge  of  the  sun's  disk,  it  has  become  pos- 
sible to  measure  the  rate  of  rotation  of  the  sun  for 
all  solar  latitudes.  Similar  studies  of  the  shifting 
of  lines  in  the  spectra  of  the  stars  in  all  parts  of  the 
heavens  indicates  towards  which  of  the  stars  the 
solar  system  is  approaching  in  its  motion  through 
space.  In  Fig.  1  of  Plate  IV  the  dark  stellar  iron 
lines  show  a  displacement  towards  the  violet  as  com- 
pared with  the  bright  iron  companion  lines  and  thus 
we  find  that  Procyon  was  approaching  the  earth  at 
the  time  when  the  observation  was  made.  In  Fig. 

1  For  additional  remarks  on  this  subject,  see  Chapter  VII. 
42 


SOLAR  INVESTIGATION 

2  of  Plate  IV  is  shown  a  pair  of  superposed  solar 
spectra  from  the  eastern  and  western  edges  of  the 
sun.  The  great  oxygen  band  called  B  gives  rise  to 
most  of  the  lines  in  this  spectral  region,  and  as  these 
lines  are  terrestrial,  not  solar,  they  are  not  displaced 
in  the  two  spectra.  But  all  the  solar  lines  show  a 
displacement,  due  to  the  fact  that  one  edge  of  the 
sun  is  approaching,  the  other  receding. 

As  regards  pressure:  The  experiments  of  Hum- 
phreys, Mohler  and  Jewell  first  showed  that  the 
spectrum  lines  of  different  elements  are  shifted 
towards  the  red  by  varying  amounts,  if  the  pressure 
of  the  surroundings  of  the  source  is  increased.  These 
pressure  shifts  are  usually  very  minute,  and  they 
follow  a  different  law  from  shifts  due  to  velocity. 
Thus  the  examination  of  the  solar  spectrum  can 
indicate  the  range  of  pressure  under  which  its 
absorption  lines  were  produced.  t 

As  regards  magnetism:  It  was  first  shown  by 
Zeeman  that  a  powerful  magnetic  field  may  split 
an  ordinary  single  spectrum  line  into  several  com- 
ponents, differing  in  the  character  of  the  polar- 
ization of  their  light  waves.  The  polariscopic  ex- 
amination of  double,  or  triple,  or  merely  widened 
spectrum  lines  may  yield  evidence  as  to  the  mag- 
netism in  the  sun.  It  is  from  such  study  that  Hale 
has  discovered  the  existence  of  magnetic  fields  in 
sun  spots.  In  most  cases  the  lines  appear  separated 
into  two  components  when  viewed  along  the  lines 
of  force  of  the  magnetic  field,  and  into  three  com- 

•l.'i 


THE  SUN 

ponents  when  viewed  at  right  angles  to  the  lines  of 
force ;  but  sometimes  four,  or  six,  or  even  more  lines 
are  seen.  In  the  simpler  cases,  first  mentioned,  the 
doublet  seen  longitudinally  consists  of  two  circularly 
polarized  rays,  one  polarized  left-,  the  other  right- 
handed.  The  triplet  seen  transversely  consists  of 
three  plane-polarized  rays,  of  which  the  central  one 
occupies  the  same  position  as  the  line  seen  in  the 
absence  of  a  magnetic  field,  and  the  two  side  rays 
occupy  the  same  positions  as  the  two  lines  seen 
longitudinally.  The  plane  of  polarization  of  the 
central  component  is  at  right  angles  to  that  of  the 
two  side  components.  Hence,  the  central  compo- 
nent may  be  extinguished  by  interposing  a  Nicol 
prism  in  a  certain  position,  while  the  two  side  com- 
ponents may  be  extinguished  and  the  central  one 
again  seen  when  the  Nicol  is  rotated  90°.  In  the 
case  of  the  doublet  seen  longitudinally,  the  two  rays 
may  be  transformed  into  plane-polarized  light  by 
introducing  a  Fresnel  rhomb.  After  this  they  may 
be  extinguished  alternately  by  a  Nicol  prism  rotated 
90°  at  a  time.  In  Plate  II,  Fig.  1  shows  a  part  of  the 
ultra-violet  spark  spectrum  of  iron  viewed  trans- 
versely to  the  lines  of  force  of  a  magnetic  field.  Figs. 
2  and  3  show  the  effects  of  interposing  the  Nicol 
prism  in  two  positions.  Fig.  1  appears  exactly  the 
same  as  the  spectrum  would  if  seen  along  the  lines 
of  force  without  the  Nicol  prism,  although,  in  fact, 
the  polarization  is  not  the  same.  Most  of  the  lines 
in  this  spectral  region  are  of  the  ordinary  type,  but 

44 


SOLAR  INVESTIGATION 

one  is  unaffected,  while  some  are  very  complex.  One 
even  has  twelve  components;  but  this  will  probably 
not  be  discerned  in  the  spectrum  as  reproduced. 

Like  the  straws  which  show  the  way  the  wind 
blows,  or  the  hieroglyphics  which  hold  the  history 
of  ancient  times,  the  lines  of  the  spectrum  yield  to 
painstaking,  minute  examination  a  wealth  of  knowl- 
edge wholly  unthought  of  by  the  careless,  and,  at 
first  glance,  unknowable.  Hardly  anything  in 
science  is  more  wonderful  than  the  extent  of  knowl- 
edge of  the  heavenly  bodies,  situated  millions, 
billions,  and  trillions  of  miles  away,  which  has  been 
acquired  through  the  spectrum. 

THE  SPECTROSCOPE 

In  common  practice  two  very  different  pieces  of 
apparatus  are  employed  to  produce  the  spectrum, 
namely  the  prism  and  the  grating ;  but  a  device  con- 
taining either  one  of  these,  in  suitable  combina- 
tion with  its  adjuncts  needed  to  produce  a  spectrum, 
is  called  a  spectroscope.  To  understand  something 
of  the  action  of  the  prism  it  is  needful  to  know  that 
light  travels  with  different  velocities  in  different 
substances,  and  that  in  general  the  different-colored 
lights  travel  with  different  velocities  in  the  same 
medium.  In  a  vacuum  all  kinds  of  light  travel  with 
equal  velocity,  and  in  most  gases,  at  ordinary 
pressures,  there  is  little  difference  for  the  different 
colors,  but  with  transparent  liquids  and  solids  the 
difference  is  very  considerable,  as  is  shown  in  the 

45 


THE  SUN 


following  little  table.     The  numbers  give  the  values 
of  the  ratio 

velocity  of  light  in  vacuo 
velocity  of  light  in  medium* 

TABLE  II. — Velocity  of  light  in  vacuo,  and  its  ratios  to  the  velocity  of 
light  in  different  media. 


Colo 

r 

Violet 

Blue 

Green 

Yellow 

Red 

Wave  length1  

4,200 

4,800 

5,400 

5,900 

6,500 

Velo 
un 

city    in    vacu- 
12  

299,860 

299,860 

299,860 

299,860 

299,860 

Ratio  of  velocity  in  vacuum 
to  velocity  in: 

Air  at   atmos- 
pheric pres- 
sure   

1.000297 

1.000295 

1.000294 

1.000293 

1.000292 

Water  

1.3420 

1.3381 

1.3354 

1.3336 

1.3320 

Average     flint 
glass  

1.6366 

1.6238 

1.6157 

1.6108 

1.6066 

Carbon         bi- 
sulphide .  .  . 

1.6835 

1.6553 

1.6374 

1.6273 

1.6188 

Diamond  

2.4570 

2.4373 

2.4242 

2.4170 

2.4108 

In  consequence  of  the  difference  of  velocity  of  light 
in  different  media,  a  beam  of  light  of  a  single  color  is 
bent  when  it  crosses  obliquely  the  boundary  between 
two  media.  This  is  illustrated  in  the  accompanying 
diagram,  Fig.  11.  The  reader  must  assume,  what  is 
shown  by  the  methods  of  physical  optics,  that  the 
direction  of  propagation  of  light  at  each  instant  is  at 

xThe  unit  of  wave  length  is  the  Angstrom,  which  is  one  ten- 
billionth  of  a  meter. 

2  The  unit  of  velocity  is  the  kilometer  per  second. 

46 


SOLAR  INVESTIGATION 


FIG.  11. 


ajb&z,  e^  cetera. 


right  angles  to  the  light  front.  A  beam  of  light, 
which  at  a  certain  instant  presents  the  front  ac,  begins 
to  enter  a  denser  medium  at  c.  In  consequence  of  the 
less  velocity  in  the  denser  medium  the  light  from  c 
moves  only  to  c1; 
while  that  from  a 
moves  to  ait  so  that 
the  lower  portion  of 
the  light  front  is 
thereby  turned  to  the 
direction  bi  c^  Suc- 
cessive positions  of 
the  light  front  are  shown  at 
Suppose  that  the  cross  section  of  the  denser  medium 
is  triangular  in  shape,  so  that  the  light  from  a4  begins 
to  proceed  more  rapidly  while  that  at  c4  is  still  being 
retarded.  This  state  of  affairs  is  that  of  the  prism  of 
the  usual  form,  and  the  light  front  finally  emerges  at 
«7c7,  proceeding  in  a  different  direction  from  that 
which  it  had  at  first.  The  difference  of  direction  de- 
pends on  the  fractional  difference  of  velocity  of  the 
ray  in  the  two  media.  Since  this  is  greater  for  violet 
light  than  for  red,  a  beam  of  light  containing  both 
colors  will  be  split  up  by  such  an  instrument,  and  the 
violet  part  will  be  more  bent  or  deviated  from  its 
original  direction  than  the  red.  Such  action  of  the 
prism  is  said  to  be  "refraction,"  the  difference  in 
direction  between  the  entering  and  emerging  beams 
is  the  "deviation,"  and  the  difference  of  direction 
between  the  different  colors  as  they  emerge  is  called 

47 


THE  SUN 

"  dispersion. "  The  angle  CcA  between  the  entering 
ray  and  the  line  perpendicular  to  the  face  of  the  prism 
is  called  the  angle  of  incidence,  and  the  angle  c7cB  the 
angle  of  refraction.  A  principal  law  of  refraction  is 
this:  The  sine  of  the  angle  of  incidence  divided  by 
the  sine  of  the  angle  of  refraction  is  constant  for  a  ray 
of  a  single  color  entering  a  given  substance,  whatever 
the  angle  at  which  it  enters.  Calling  i  the  angle  of 
incidence,  r  the  angle  of  refraction,  and  n  the  con- 
stant, or  index  of  refraction: 

sin  i 

n  =  -7 — . 
sin  r 

The  value  of  the  index  of  refraction  is,  therefore,  a 
matter  of  much  importance  for  calculation,  and  it  is 
still  more  interesting  because  it  is  also  the  ratio  of  the 
velocities  of  the  light  in  the  two  media  concerned. 
For  yellow  light  arid  for  ordinary  telescope  flint  glass 
and  air  as  the  two  media,  the  refractive  index  is  about 
1.61  (see  Table  II). 

It  follows  mathematically  from  the  law  just  stated, 
that  for  a  given  prism  and  a  given  color  of  light  the 
deviation  can  never  fall  below  a  certain  minimum 
value,  whatever  the  angle  of  incidence.  This  small- 
est angle  of  deviation  is  called  the  angle  of  minimum 
deviation,  and  is  secured  when  the  angle  of  incidence 
is  equal  to  the  angle  of  emergence.  For  this  position 
of  the  prism  the  following  relation  holds,  if  we  desig- 
nate the  angles  of  incidence  and  deviation  as  i  and 
Z>,  and  the  angle  at  the  apex  of  the  prism  as  A : 
D=2i-A. 
48 


SOLAR  INVESTIGATION 

In  the  use  of  prismatic  spectroscopes  it  is  generally 
preferable  that  the  rays  of  light  composing  the  beam 
shall  be  parallel  to  one  another  as  they  enter  the 
prism,  and  that  the  prism  shall  be  set  for  minimum 
deviation.  The  beam  which  emerges  when  white 
light  passes  the  prism  under  these  circumstances  con- 
sists of  a  mixture  of  bundles  of  parallel  rays  of  light  of 
different  colors,  with  the  neighboring  shades  differing 
by  almost  imperceptible  inclinations  one  from  an- 
other in  their  paths  of  emergence.  It  is  necessary  to 
bring  them  to  focus  by  means  of  a  lens  or  mirror  if 
they  are  to  be  sharply  separated.  If  the  light  comes 
originally  from  a  star  the  rays  will  be  practically  par- 
allel without  alteration;  but  if  they  come  from  the 
sun  they  converge  from  opposite  sides  Of  the  solar 
disk  with  an  angle  of  over  30'  of  arc.  For  solar  work, 
therefore,  and  often  for  stellar  wrork,  also,  two  other 
adjuncts  to  the  prism  are  used.  The  first  is  a  narrow 
slit  between  sharp  metal  jaws  parallel  to  the  line  of 
intersection  of  the  prism  faces,  and  the  second  is  a 
lens  or  mirror  placed  at  such  a  distance  as  to  render 
the  rays  which  diverge  from  the  slit  parallel.  A  lens 
or  mirror  serving  the  latter  purpose  is  called  a 
collimator,  and  the  lens  or  mirror  which  focuses  the 
spectrum  is  called  the  objective,  or  image-forming 
piece.  This  arrangement  is  indicated  in  Fig.  12. 

With  most  prisms  the  violet  part  of  the  spectrum  is 
much  more  extended  than  the  red,  owing  to  the  more 
rapid  proportional  variation  of  the  velocity  of  light  in 
glass  at  the  violet  end  of  the  spectrum. 

49 


THE  SUN 


The  grating,  as  a  means  of  dispersing  light,  de- 
pends on  the  phenomenon  called  interference.  Light, 
like  sound,  is  propagated  by  wave  motions.  If  a 
tuning  fork  be  set  in  vibration  and  slowly  rotated 
while  held  in  the  hand  at  some  distance  from  the  ear, 
the  sound  will  be  found  to  wax  and  wane  in  loudness, 


A         COLLIMATOR 


FIG.  12. 


although  the  fork  continues  to  vibrate  steadily.  The 
position  of  faint  sound  occurs  because  the  vibration 
of  air  excited  by  one  prong  of  the  fork  reaches  the  ear 
so  much  later  than  that  excited  by  the  other  that, 
while  one  wave  is  in  what  corresponds  to  a  crest,  the 
other  is  in  what  corresponds  to  a  trough.  At  all 
parts  of  the  waves  their  effects  are  similarly  opposed, 
and  the  result,  if  the  waves  are  equal  in  strength,  is 
silence. 

With  light  a  similar  thing  may  occur.  From  two 
slits,  ax  and  a2  (Fig.  13),  imagine  light  of  a  single 
color  to  proceed  in  all  directions.  Then  at  biy  b2,  etc., 
the  waves  may  be  supposed  to  arrive  in  opposition, 
and  thus  to  produce  darkness,  while  at  c,,  c2,  c3, 
etc.,  there  is  light.  From  a  great  number  of  other 

50 


SOLAR  INVESTIGATION 

slits,  a3  a4,  etc.,  placed  in  a  plane,  and  equally 
spaced,  the  directions  of  light  and  darkness  will  be 
the  same,  so  that  if  a  piece  of  plane  glass  is  coated 
with  silver,  and  the  silver  coat  is  scratched  off 
in  a  series  of  parallel  and  equidistant  lines,  a  bundle 
of  rays  of  light  passing  through  the  slits  will  some 
of  them  proceed  parallel  to  the  direction  AB,  while 
the  others  will  be  deviated,  or  diffracted,  as  it  is 
called,  in  various  definite  directions  on  either  side  of 
the  central  beam.  These  directions  depend  upon  the 


ll 
r3                            w 

r                        c 

w 

1*                        1 

/// 

I 

n 

B 


FIG.  13. 

interval  between  successive  rulings,  and  on  the  length 
of  the  wave  of  the  given  color  of  light.  The  devia- 
tions are  less  for  violet  light  than  for  the  red,  which 
shows  that  the  length  of  wave  is  less  for  violet  rays. 
Such  a  grating,  as  has  just  been  mentioned,  is 
called  a  transmission  grating,  but  it  is  more  common 
to  employ  a  reflecting  grating.  A  carefully  ground 
and  polished  surface  of  speculum  metal  is  scratched 
with  a  diamond  point  in  parallel  rulings  very  close  to- 
gether, not  uncommonly  as  many  as  20,000  to  the 

51 


THE  SUN 

inch.  One  may  suppose  that  with  such  close  ruling, 
the  spaces  between  the  ruled  scratches  are  probably 
like  the  rough  ridges  turned  up  by  a  plow,  and,  as 
they  would  reflect  but  weakly,  they  may  be  assumed 
to  correspond  to  the  opaque  parts  of  the  transmission 
grating,  while  the  smooth  sides  of  the  scratches  act  as 
bright  sources  of  light.  To  the  late  Prof.  Henry  A. 
Rowland,  of  Baltimore,  is  due  the  principal  share 
of  the  credit  for  the  great  advance  in  knowledge  of  the 
solar  spectrum,  and  of  that  of  the  spectra  of  vaporized 
substances,  which  has  come  in  the  last  twenty-five 
years;  for  he  it  was  who  designed  the  perfected  screw, 
and  thereby  was  enabled  to  construct  hitherto  un- 
equaled  ruling  machines.  Rowland  gratings,  having 
a  total  of  as  many  as  60,000  lines  or  more,  each  two 
or  three  inches  long,  are  in  nearly  every  large  labo- 
ratory and  observatory  of  the  world.  Not  only  did 
he  thus  promote  the  work  of  others,  but  his  own  em- 
ployment of  his  gratings  has  left  some  branches  of 
solar  spectroscopy  at  the  furthest  forward  mark  they 
have  yet  reached. 

Diffraction  gratings  may  be  ruled  on  flat  surfaces 
and  used  with  a  collimator  and  objective  like  a  prism, 
but  many  of  them  are  ruled  on  concave  surfaces,  and 
are  used,  after  a  design  of  Rowland,  without  collima- 
tor or  objective.  Thus,  we  have  the  plane  grating 
and  the  concave  grating  spectroscopes.  The  arrange- 
ment of  the  former  is  shown  in  Fig.  14.  Frequently, 
however,  the  collimator  is  used  also  as  the  image- 
forming  lens.  Such  an  arrangement  is  called  the 

52 


SOLAR  INVESTIGATION 

Littrow  form  of  spectroscope.  It  may  be  employed 
also,  for  prismatic  instruments  if  a  plane  mirror  is  in- 
troduced to  return  the  beam  through  the  prism.  It 
is  necessary  to  tip  the  grating  or  mirror  a  little  so 
that  the  spectrum  is  formed  above  or  below  the  slit. 
Fig.  15  shows  the  concave  grating  arrangement^ 
In  this  figure,  S  is  the  slit,  G  the  grating,  and  I  the 
spectrum.  C  is  a  rigid  bar  which  carries  the  grating 


SLIT 


FIG.  14. 

and  the  means  of  observing  the  spectrum.  "This  bar, 
mounted  on  carriages,  k  and  k',  slides  over  the  tracks, 
R  and  R'.  These  tracks  are  placed  at  right  angles, 
with  their  intersection  at  S. 

From  a  white  light  a  grating  spectroscope  produces 
a  series  of  spectra  more  and  more  diverging  on  either 
side  of  a  single  white  band  in  the  center.  These  spec- 
tra are  called  first,  second  and  third  order,  et  cetera, 
according  to  their  divergence.  Only  one  spectrum  can 
be  made  use  of  at  a  time  among  all  this  multitude, 
and  the  greater  the  number  of  the  order,  the  greater 
the  dispersion  of  the  spectrum.  The  higher  orders 
overlap,  so  that  the  red  of  one  order  falls  on  the  violet 
or  some  other  color  of  the  next  higher  order.  When  it 

53 


THE  SUN 

is  necessary  to  separate  entirely  one  color  from  the 
other,  it  is  customary  to  interpose  somewhere  in  the 
beam  an  absorbing  screen  which  is  opaque  to  the 
color  not  desired,  but  transparent  to  the  other.  Spec- 


FIG.  15. 

tra  of  very  high  orders,  however,  are  hopelessly  mixed 
and  are  practically  white  light.  It  is  seldom  that 
spectra  above  the  fourth  order  are  employed.  The 
relative  brightness  of  grating  spectra  depends  on  the 

54 


SOLAR  INVESTIGATION 

form  of  the  grooves  ruled.  Some  diamond  points 
produce  gratings  very  bright  in  one  or  two  particular 
spectra,  and  are  preferred  for  this  reason.  The  selec- 
tion of  a  good  diamond  point  is  the  result  usually 
of  trial  rather  than  of  microscopic  examination.  A 
spectrum  may  be  very  bright  for  some  colors  and  not 
for  others.  At  best,  a  grating  seldom  throws  as 
much  as  one-tenth  of  the  light  into  one  spectrum, 
and,  therefore,  in  researches  where  loss  of  light  is  very 
serious  a  prism  is  often  preferred,  since  it  may  trans- 
mit as  much  as  eighty-five  per  cent.  In  a  prismatic 
spectrum  the  violet  is  greatly  extended  as  compared 
with  the  red,  while  in  a  concave  grating  spectrum  the 
dispersion  is  a  linear  function  of  the  wave  length. 
That  is  to  say>  equal  distances  along  the  concave- 
grating  spectrum  correspond  to  equal  differences  of 
wave  length.  Such  spectra  are  said  to  be  "normal." 
A  plane-grating  spectrum  is  nearly  normal  for  short 
distances. 

The  wave  lengths  in  the  spectrum,  as  visible  to  the 
eye,  range  from  about  0.39/z1  to  0.80/z.  Beyond  the 
violet  the  solar  spectrum  runs  to  a  wave  length  of 
0.29/4,  where  it  is  practically  cut  off,  partly  by  the 
nontransparency  of  our  own  atmosphere  (particu- 
larly the  nontransparency  of  ozone)  and  perhaps 
imperatively  by  the  opaqueness  of  the  solar  envelope. 
Beyond  the  red  the  solar  spectrum  extends  to  a  wave 
length  of  about  20/x,  though  with  several  long  inter- 

1  The  micron,  or  thousandth  of  a  millimeter,  is  denoted  by  the 
Greek  letter  |t. 

6  55 


THE  SUN 

missions  due  to  the  nontransparency  of  the  atmos- 
phere (especially  of  water  vapor,  carbonic  acid  and 
ozone),  from  which  cause  it  practically  ceases  at 
20//,.  Ordinary  glass  apparatus  ceases  to  be  trans- 
parent at  about  wave  length  0.35//,  in  the  ultra-violet, 
and  at  about  2.5//,  in  the  infra-red,  but  the  limits 
differ  with  different  kinds  of  glass.  Quartz  apparatus 
is  transparent  to  rays  of  all  wave  lengths  from  less 
than  O^Oyit1  to  more  than  4.0u.  Fluorite  is  transparent 
in  the  ultra-violet,  and  in  the  infra-red  its  transpar- 
ency extends  to  about  7. Op.  Rock-salt  is  also  trans- 
parent in  the  ultra-violet,  and  as  far  as  17/A  in  the 
infra-red.  Silvered  glass  mirrors  reflect  almost  totally 
for  all  rays  of  the  infra-red  and  visible  spectrum,  and 
their  reflecting  power  remains  high  as  far  as  wave 
length  0.33ya  in  the  ultra-violet.  Between  wave 
lengths  0.33/1,  and  0.29/*,  the  reflecting  power  of  silver 
does  not  reach  fifteen  per  cent.  Speculum  metal, 
which  is  used  for  gratings,  reflects  much  less  strongly 
than  silver  in  the  visible  spectrum,  but  continues  to 
reflect  forty  per  cent  or  more  to  beyond  wave  length 
0.30/A. 

As  stated  above,  it  is  the  minute  study  of  the  lines 
found  in  spectra  which  yields  many  of  the  most  inter- 
esting results,  and  in  the  solar  spectrum  these  lines 
become  increasingly  numerous  towards  the  violet, 
and  in  the  ultra-violet.  Fortunately,  the  ordinary 

1  Although  solar  rays  of  less  wave  length  than  0.29ft  are  not  found, 
terrestrial  sources  give  rays  of  much  shorter  wave  lengths,  even  to 

o.ion 

56 


SOLAR  INVESTIGATION 

photographic  plate  is  highly  sensitive  in  this  thickly 
lined  violet  and  ultra-violet  part  of  the  spectrum,  and 
at  present  most  spectrum  investigations  are  made 
photographically.  There  are  special  photographic 
plates  which  are  sensitive  in  other  parts  of  the  spec- 
trum. By  staining  ordinary  plates  with  certain  dyes, 
they  may  be  employed  for  red  rays,  and  even  a  little 
beyond  the  visible  limit  of  the  red  spectrum.  For 
spectrum  investigations  far  beyond  the  red,  it  is 
necessary  to  use  sensitive  heat  measuring  apparatus, 
such  as  will  soon  be  described. 

For  some  purposes  it  is  sufficient  to  allow  the  rays 
of  the  sun  to  shine  directly  into  the  spectroscope, 
but  ordinarily  it  is  necessary  to  confine  the  obser- 
vations to  selected  areas  of  the  sun  such  as  a  sun 
spot,  or  to  the  sun's  edge  or  "limb"  as  distinguished 
from  the  center.  To  do  this  the  slit  of  the  spec- 
troscope must  be  placed  in  the  focus  of  a  lens  or  con- 
cave mirror  which  forms  a  solar  image  of  suitable 
dimensions  for  the  investigation.  When  the  spec- 
troscope is  large,  and  the  work  requires  it  to  be  main- 
tained at  perfectly  constant  temperature  for  long 
photographic  exposures,  it  becomes  highly  desirable 
to  keep  the  spectroscope  fixed  and  to  employ  a 
ccelostat  to  reflect  light  to  the  lens  or  mirror.  Fig. 
16  shows  the  new  150-foot  tower  telescope,  with  a  pit 
75  feet  deep  beneath  for  the  spectroscope,  as  just 
being  completed  at  the  Mount  Wilson  Solar  Obser- 
vatory. A  smaller  tower  telescope  has  been  doing 
good  work  there  for  a  considerable  time.  The 

57 


THE  SUN 


coelostat  is  on  the  top  of  the  tower  60  feet  high,  and 
reflects  a  beam  of  sunlight   vertically   downwards 

through  a  lens  which  forms 
a  solar  image  over  7  inches 
in  diameter  upon  the  slit  of 
the  spectroscope  near  the 
surface  of  the  ground.  The 
slit  is  in  the  center  of  a  turn- 
table which  supports,  by 
rigid  steel  construction,  the 
collimator  and  plane  grating 
30  feet  below  ground.  Thus, 
the  whole  spectroscope  can 
be  rotated  about  the  axis  of 
the  beam  of  light.  The  col- 
limating  lens  acts,  also,  as 
an  image-forming  lens  (the 
Littrow  type  of  spectro- 
scope), and  the  spectrum 
falls  on  the  photographic 
plate  fixed  upon  the  surface 
IS'  of  the  turntable  near  the  slit. 
Below  ground  the  tempera- 
ture is  very  constant.  At 
the  top  of  the  tower,  the  air 
is  nearly  free  from  the  trem- 
ors which  cause  " boiling"  of 
the  image.  As  the  beam  de- 
scends vertically  from  the 
top  of  the  tower  it  is  less 
58 


FIG.  16. 


SOLAR   INVESTIGATION 

likely  to  be  distorted  by  "boiling"  than  it  would  be  if 
coming  obliquely,  as  from  the  sun  directly.  Hence, 
altogether,  the  tower  plan  of  solar  observatory  is 
highly  favorable  for  carrying  on  exact  investigations 
with  powerful  apparatus.  The  new  tower  telescope 
of  over  150  feet  focus  just  being  erected  for  the  Mount 
Wilson  Solar  Observatory  will  doubtless  yield  very 
remarkable  results. 

THE  SPECTROHELIOGRAPH 

The  spectroheliograph,  invented  by  Dr.  G.  E. 
Hale,  is  a  device  for  photographing  the  sun  in  the 
light  of  a  single  wave  length.  Let  us  suppose  that 
the  solar  image  is  brought  to  focus  on  the  slit  of  a 
spectroscope,  and  that  the  slit  is  longer  than  the 
diameter  of  the  image.  The  spectroscope  may  be 
adjusted  so  that  a  certain  Fraunhofer  line,  perhaps 
the  line  called  C,  or  otherwise  Ha  (due  to  hydrogen), 
falls  in  the  center  of  the  field  of  view.  Then,  if  the 
solar  image  is  allowed  to  drift  across  the  slit,  the 
observer  will  see  the  masses  of  hydrogen  on  the  sun 
which  emit  the  light  in  question  as  their  images  pass 
in  succession  over  the  slit.  But  it  would  be  practically 
impossible  to  note  and  remember  or  sketch  these 
details.  If  the  photographic  plate  is  substituted 
for  the  eye,  and  a  slit  placed  just  in  front  of  it,  so 
narrow  as  to  permit  only  the  Ha  line  to  pass,  a 
photographic  record  would  be  made,  but  this  would 
be  a  mixture  of  all  the  successive  views  of  the  hydro- 
gen masses,  and  would  be  useless.  But  by  moving 

59 


THE  SUN 

the  plate  along  at  the  same  rate  that  the  image  of 
the  sun  drifts,  there  would  be  a  new  part  exposed  for 
every  succeeding  impression,  and  the  result  would  be 
a  photograph  of  the  hydrogen  masses  which  emit 
Ha  light,  as  they  exist  over  the  whole  sun's  disk. 
This  is  one  form  of  the  spectroheliograph.  In 
another  form,  which  is  employed  for  the  five-foot 
spectroheliograph  of  the  Snow  telescope  of  the 
Mount  Wilson  Solar  Observatory,  the  whole  spec- 
troscope is  floated  on  mercury,  and  moved  slowly,  at 
right  angles  to  the  beam,  across  the  sun's  image  and 
the  photographic  plate,  both  of  which  remain  station- 
ary. The  solar  image  of  the  Snow  telescope  is  about 
7  inches  in  diameter,  and,  if  the  correspondingly  long 
slit  of  the  spectroscope  were  straight,  the  spectrum 
lines  would  be  greatly  curved,  and  the  sun's  image 
taken  with  the  spectroheliograph  would  be  distorted. 
This  defect  is  avoided  by  using  curved  slits,  dividing 
the  necessary  curvature  between  that  for  the  spectro- 
scope and  that  in  front  of  the  plate.  The  curvature 
of  these  slits  differs  for  different  spectrum  lines,  so 
that  as  many  pairs  of  slits  are  required  as  there  are 
spectrum  lines  in  which  spectroheliograms  are  de- 
sired. Thus  far  Ha,  H/3,  Hy,  HS  of  hydrogen,  H  and 
K  of  calcium,  and  a  few  preliminary  tests  of  other 
lines  have  been  tried. 

THE  HELIOMICROMETER 

It  ordinarily  requires  considerable  measurement 
and  calculation  to  determine  the  positions  of  objects 

60 


SOLAR  INVESTIGATION 

with  reference  to  the  solar  equator,  seen  on  the  solar 
photographs,  whether  direct  or  spectroheliographic. 
This  labor  is  largely  avoided  by  the  use  of  a  device 
of  Mr.  Hale's  called  the  heliomicrometer.  It  con- 
sists of  a  sphere  marked  with  circles  of  latitude  and 
longitude,  and  adjusted  so  that  its  poles  correspond 
in  position  with  those  of  the  sun  for  the  date  in 
question.  A  long  focus  concave  mirror  throws  an 
image  of  this  sphere,  and  of  the  photographic  plate 
to  be  examined,  simultaneously  into  a  double-field 
eyepiece.  Thereby,  the  two  images  are  superposed, 
and  the  observer  sees  the  solar  photograph  appar- 
ently marked  with  lines  of  latitude  and  longitude 
corresponding  with  those  of  the  sun.  A  micrometer 
is  provided  for  accurately  measuring  the  distances 
of  the  images  of  the  solar  objects  from  the  nearest 
reference  lines. 

THE  COMPARATOR 

In  all  photographic  spectrum  work,  the  main 
thing  is  accurate  measurements  of  the  positions  of 
the  spectrum  lines  with  reference  to  each  other, 
or  with  reference  to  certain  standards  of  position. 
In  many  cases  the  slit  of  the  spectroscope  is  partly 
covered  by  a  diaphragm  of  peculiar  shape,  which 
can  be  moved  so  as  to  uncover  different  portions  of 
the  slit.  Thus,  successive  exposures  may  be  made 
to  different  sources  of  light,  as,  for  instance,  the 
center  and  limb  of  the  sun,  or  the  sun  and  the 
iron  arc  light.  In  the  resulting  photograph  there 

61 


THE  SUN 

are  several  spectra  corresponding  to  these  different 
sources,  all  accurately  aligned  one  above  another. 
For  measurement,  the  photograph  is  placed  on  the 
table  of  a  measuring  machine,  or  comparator,  and 
this  table  is  moved  to  and  fro  by  an  accurate  screw 
with  graduated  head,  thus  bringing  chosen  spectrum 
lines  to  the  cross  hair  of  the  observing  microscope. 
Measurements  of  position  to  the  ten-thousandth 

part   of  a  millimeter  (to  inch)     are   some- 


times  made  in  this  manner. 

The  wave  lengths  of  the  solar  spectrum  lines  and 
of  the  bright  spectrum  lines  of  the  chemical  elements 
are  the  fundamental  data  of  spectroscopy.  In  Row- 
land's great  table  of  the  solar  spectrum  the  wave 
lengths  are  given  to  seven  places  of  significant  figures, 
that  is,  to  thousandths  of  an  "Angstrom  unit."  It 
has  lately  been  found  that  there  are  certain  syste- 
matic errors  of  the  table  due  to  various  causes, 
chiefly  to  an  obscure  source  of  error  in  the  use  of  the 
grating  for  determining  wave  lengths,  so  that  there 
are  corrections  of  the  order  of  one  or  two  hundredths 
of  an  Angstrom  to  be  applied  to  make  Rowland's 
table  homogeneous.  To  reduce  to  the  absolute 
scale  of  the  international  metric  system,  a  some- 
what larger  correction  is  needed.  By  means  of  the 
interferometer  these  corrections  are  gradually  being 
determined,  and  it  is  probable  that  within  a  few 
years  we  shall  have  a  standard  table  of  solar  and 
terrestrial  spectrum  places  accurate  to  within  two  or 

62 


SOLAR  INVESTIGATION 

three  units  in  the  seventh  place  of  significant  figures. 
It  seems  extraordinary  enough  that  so  small  a  quan- 
tity as  the  wave  length  of  light  should  be  measurable 
to  such  extreme  precision,  and  still  more  extraor- 
dinary that  such  a  decree  of  accuracy  is  at  all 
necessary  for  promoting  investigation.  But  so  it 
is,  and  much  of  the  remarkable  progress  of  solar 
knowledge  in  recent  years  depends  on  differences  of 
wave  lengths,  as  in  the  case  of  pressure  and  velocity 
shifts,  not  larger  than  0.005  of  an  Angstrom,  or  less 
than  one-millionth  part  of  the  wave  length  of  yellow 

light. 

THE  NATURE  OF  RADIATION 

Not  less  important,  perhaps,  than  these  questions 
of  exact  wave  lengths,  is  the  measurement  of  the 
intensity  of  light,  or  rather,  speaking  more  broadly, 
of  radiation.  All  solar  rays,  whether  visible  or 
photographically  active  or  not,  produce  heat  when 
absorbed  upon  a  blackened  surface.  Sometimes  the 
infra-red  rays  are  called  "heat  rays,"  the  light  rays, 
"  visible  rays/'  and  the  blue,  violet  and  ultra-violet, 
"actinic,"  or  " photographic  rays."  But  there  is  no 
distinction  of  kind  between  these  things.  All  are 
regarded  as  transverse  vibrations  of  the  luminifer- 
ous  ether,  differing  only  in  wave  length.  Just  as 
there  are  sound  waves  too  high  or  too  low  in  pitch 
to  be  heard,  so  radiation  may  be  too  long  or  too  short 
in  wave  length  to  be  seen,  but  this  implies  no  dif- 
ference in  kind  of  vibration. 

The  intensity  of  radiation  can  be  quantitatively 

63 


THE  SUN 

estimated  only  very  imperfectly  by  the  eye,  or  by 
the  aid  of  the  photographic  plate,  although  both  the 
eye  and  the  plate  are  excessively  sensitive  to  radia- 
tion of  certain  wave  lengths.  But  waves  of  all 
wave  lengths  produce  their  just  effects  when  trans- 
formed into  heat.  Though  both  are  forms  of  energy, 
radiation  is  not  heat,  but  may  be  transformed  com- 
pletely into  heat.  We  regard  radiation  as  wave  mo- 
tion in  the  ether,  heat  as  irregular  motion  of  the 
molecules  of  material  substances.  All  heated  sub- 
stances give  off  radiation;  but  the  amount  and  quality 
of  radiation  given  off  at  a  given  temperature  are 
different  for  different  substances.  Substances  at 
any  temperature  above  the  absolute  zero  ( —273°  C.) 
are  supposed  to  consist  of  molecules  in  rapid  mo- 
tion. These  moving  molecules  may  be  supposed 
to  communicate  some  of  their  energy  to  the  unseen 
ether  which  is  assumed  to  permeate  all  space,  even 
the  interstices  between  the  molecules  of  solid  bodies. 
Thereby  the  ether  may  be  assumed  to  be  set  in  con- 
fused vibration,  and  from  this  confusion  is  extri- 
cated by  the  prism,  or  grating,  the  orderly  succession 
of  wave  lengths  which  we  term  the  spectrum.  The 
relative  intensity  of  the  several  parts  of  such  a 
spectrum  depends  on  the  temperature  of  the  exciting 
body. 

Kirchhoff  introduced  the  notion  of  the  perfect 
radiator.  This  is  sometimes  called  "the  absolutely 
black  body/'  because  a  perfect  radiator  is  a  per- 
fect absorber  of  radiation,  and  most  black  substances 

64 


SOLAR   INVESTIGATION 

are  also  nearly  perfect  absorbers.  The  perfect  radi- 
ator emits  for  a  given  temperature  the  maximum 
possible  amount  of  radiation  of  each  and  every 
wave  length;  so  that  no  other  body  at  the  same 
temperature  can  excel  its  emission  for  any  wave 
length.1 

LAWS  OF  RADIATION 

Kirchhoff  proved  the  following  important  rela- 
tion, now  known  as  Kirchhoff 's  law:  For  any  given 
temperature  and  wave  length  the  ratio  of  the  emis- 
sion of  a  body  to  its  absorption  is  a  constant,  and 
equal  to  the  emission  of  a  perfect  radiator  for  the 
same  temperature  and  wave  length.  In  order  to 
understand  this  law,  the  force  of  the  expressions 
emission  and  absorption  must  be  clearly  grasped. 
By  emission  is  meant  the  rate  of  escape  of  energy  by 
radiation,  and  to  fix  ideas  it  may  be  regarded  as  the 
amount  radiated  from  each  square  centimeter  of  sur- 
face in  a  minute  of  time.  By  absorption  is  meant 
the  fraction  which  would  be  absorbed  in  the  body 
if  sbined  upon  by  radiation  from  another  source. 
For  instance,  if  thus  shined  upon,  and  three-fourths 
of  the  rays  received  are  absorbed  and  go  to  warm 
the  body,  while  the  other  fourth  is  reflected  away, 
or  transmitted,  the  absorption  is  said  to  be  three- 
fourths.  Such  a  body,  by  Kirchhoff's  law,  would 
emit  only  three-fourths  as  copiously,  for  the  wave 

1  An  exception  must  be  made,  perhaps,  of  a  certain  class  of  bodies 
excited  to  radiation  by  other  causes  than  temperature,  as,  for  in- 
stance, chemical  action.  The  remarks  above  concern  the  relations 
of  temperature  and  radiation  alone. 

65 


THE   SUN 

length  and  temperature  in  question,  as  would  the 
perfect  radiator. 

The  importance  of  the  conception  of  the  perfect 
radiator  will  appear  as  we  go  on.  No  substance  in 
the  world  answers  to  its  requirements,  but  lamp- 
black is  very  nearly  a  perfect  radiator  at  low  tem- 
peratures. However,  if  a  closed  hollow  chamber  is 
formed  of  any  substance  whatever,  and  its  walls 
maintained  at  uniform  temperature,  the  radiation  in- 
side the  chamber  will  be  that  of  the  perfect  radiator. 
If  a  small  hole  be  made  in  the  wall  the  radiation 
which  escapes  through  the  hole  will  be  practically 
perfect  radiation.  Instruments  of  this  form  have 
been  constructed  within  the  last  fifteen  years,  and 
careful  measurements  have  been  made  of  the  inten- 
sity of  their  emission  for  a  great  range  of  wave 
lengths,  and  for  temperatures  from  that  of  liquid  air 
up  to  that  of  melting  platinum.  These  results  have 
been  compared  with  the  theoretical  radiation  formu- 
lae connecting  temperature,  wave  length  and  radia- 
tion which  have  been  proposed.  L/ 

The  formula  of  Wien,  as  modified  by  Planck,  is 
found  to  express  the  observed  results.  Let  e  be  the 
emission  of  wave  length  X  by  the  perfect  radiator  of 
the  temperature  T,  and  let  e  be  the  base  of  the  Napier- 
ian system  of  logarithms,  and  let  cx  and  c2  be  two  con- 
stants determined  by  experiment.  Then: 

e  =  c^-5(e **- 1)'1     (The  Wien-Planck  formula).     I 

As  stated  above,  no  body  emitting  rays  by  virtue  of 

66 


SOLAR  INVESTIGATION 

temperature  can  exceed  the  radiation  determined  by 
this  formula  for  any  wave  length  or  temperature. 

Another  formula  of  nearly  equal  importance,  due 
to  Stefan,  gives  the  measure  of  the  sum  total  of  radi- 
ation, E,  of  all  wave  lengths,  for  a  perfect  radiator  of 
the  absolute  temperature,  T.  It  is  this : 

E  =  o-T4  (Stefan's  formula).     II 

The  quantity,  <r,  is  a  constant  determined  by  experi- 
ment. 

A  third  formula,  called  Wien's  displacement  law, 
connecting  the  wave  length  of  maximum  emission, 
^-max.  (expressed  in  thousandths  of  a  millimeter,  or  /&), 
with  the  absolute  temperature  T  is  as  follows: 

A-max.  T  =  2930    (Wien's  displacement  formula).      Ill 

It  is  from  these  three  formulae  that  we  are  able  to 
obtain  some  definite  ideas  of  the  minimum  tempera- 
ture of  the  sun.  Many  bodies  appear  to  approach 
the  state  of  being  perfect  radiators  at  high  tempera- 
tures, although  departing  greatly  from  it  at  low  tem- 
peratures. But  no  body  radiating  by  virtue  of  its 
temperature  can  excel,  either  in  the  sum  total  of  its 
radiation,  or  in  that  of  any  wave  length,  the  emission 
of  the  perfect  radiator  of  the  same  temperature. 
Hence,  if  we  can  determine  by  Formula  II  the  temper- 
ature which  the  perfect  radiator  would  have  in  order 
that  its  radiation  should  approximate  in  quantity 
the  emission  of  the  sun,  then  it  is  sure  that  the  solar 
temperature  must  be  as  high  or  higher. 

Before  giving  the  values  of  the  constants  in  these 

67 


THE  SUN 

'formulae,  we  must  consider  how  energy  of  radiation 
can  be  measured.  There  are  no  accurate  means  of 
measuring  radiant  energy  while  it  remains  such.  It 
must  first  be  transformed  into  heat.  The  unit  of 
measurement  of  heat  is  the  calorie,  or  that  amount  of 
heat  which  is  required  to  warm  one  gram  of  water  at 
15°  C.  through  one  degree.  With  this  unit  we  must 
combine  the  notion  of  intensity.  We  then  define 
the  unit  intensity  of  radiant  energy  as  that  which, 
if  completely  absorbed  by  a  surface  at  right  angles 
to  the  beam,  will  produce  one  calorie  of  heat  per 
square  centimeter  per  minute.  We  therefore  meas- 
ure radiation  in  calories  per  square  centimeter  per 
minute. 

To  suit  this  definition,  and  to  correspond  with 
wave  lengths  expressed  in  microns  (A6),  and  tem- 
peratures in  absolute  degrees  of  the  Centigrade  scale, 
the  values  of  the  constants  of  formulae  I  and  II  are  as 
follows : 

C,=  5.29  X  105;     C2  =  14,550;     a-  =  76.8  X  10'12. 

SPECTRA  OF  DIFFERENT  SOURCES 

In  Fig.  17  the  curves  A  and  B  give  the  distribution 
of  radiation  in  the  spectrum  of  a  perfect  radiator  at 
7000°  and  6200°  of  the  absolute  Centigrade  tempera- 
ture scale,  as  computed  from  the  Formula  I.  The 
curve  C  gives  the  distribution  of  radiation  as  it  would 
be  found  in  the  average  spectrum  of  the  sun's  entire 
disk,  if  it  could  be  observed  outside  of  our  atmos- 
phere, according  to  determinations  made  by  Smith- 

68 


SOLAR  INVESTIGATION 


1L 


a:  AE3S 


30   v 


ooc 


sonian  expeditions  on  the  summits  of  Mount  Wilson 
and  Mount  Whitney.  The  wave  lengths  are  given 
by  the  horizontal  distances  (abscissae)  and  are  in 
thousandths  of  a  millimeter,  or  microns,  usually  de- 

69 


THE  SUN 

noted  by  the  Greek  letter  /*.  The  visible  spectrum 
practically  extends  from  0.4^  to  0.7/*1,  so  that  much 
of  the  solar  radiation  is  invisible.  The  vertical  heights 
of  the  curves  (ordinates)  are  proportional  to  the  energy 
of  the  rays  of  corresponding  wave  lengths  as  meas- 
ured by  their  heating  effects.  It  will  be  noted  that 
the  forms  of  the  computed  and  observed  curves  differ 
most  in  the  ultra-violet,  where  the  observed  solar 
radiation  falls  off  more  rapidly  than  the  computed 
radiation  of  the  perfect  radiator.  Further  remarks 
on  the  subject  of  the  sun's  temperature  will  be  given 
in  the  next  chapter. 

The  reader  will  note  that  the  maximum  ordiriate  of 
curve  A  occurs  at  a  less  wave  length  than  that  for 
curve  B,  and  that  curve  A  is  at  all  points  the  higher 
of  the  two.  The  perfect  radiator  is  supposed  to  emit 
rays  of  all  wave  lengths  at  all  temperatures,  whether 
high  or  low;  but  when  the  temperature  is  low  the 
shorter  wave  lengths,  including  those  which  would  be 
visible,  are  too  weak  to  be  detected,  even  by  such  a 
highly  sensitive  organ  as  the  eye.  As  the  tempera- 
ture increases  the  intensities  of  rays  of  all  wave 
lengths  increase,  but  the  intensities  of  rays  of  shorter 
wave  length  increase  most  rapidly.  Hence,  as  ex- 
pressed in  Formula  III,  the  wave  length  of  the 
maximum  emission  grows  less  and  shifts  towards  the 
violet  end  of  the  spectrum  as  the  temperature  in- 

1  By  special  devices,  the  spectrum  can  be  observed  visually  from 
0.37  fji.  to  0.83  /*,  but  as  ordinarily  observed  it  falls  within  the  limits 
above  stated. 

70 


SOLAR  INVESTIGATION 

creases.  Most  common  solids  and  liquids  emit  a 
continuous  spectrum,  which,  as  the  temperature  in- 
creases, grows  in  intensity  more  rapidly  for  short 
wave  lengths  than  for  long.  But  there  are  usually 
special  regions,  or  bands  of  the  spectra  of  solids  and 
liquids  where  the  radiation  is  stronger  than  that  of 
the  adjacent  wave  lengths.  These  are  called  regions 
of  "selective  emission/'  and,  as  follows  from  Kirch- 
hoff's  law,  they  are  also  regions  of  "  selective  absorp- 
tion." 

When  gases  or  vapors  are  examined  under  ordinary 
conditions  of  low  pressure,  and  with  small  quantity 
present,  as  when  the  electric  arc  is  caused  to  play 
between  metallic  poles,  the  spectrum  appears  to  be 
made  up  chiefly  of  narrow  lines  or  bands  of  selective 
emission,  without  a  prominent  accompanying  con- 
tinuous spectrum.  Some  authors  hold  that  the  con- 
tinuous background  is  totally  absent  in  gaseous  spec- 
tra, but  it  seems  more  likely  that  there  is,  in  fact,  a 
very  slight  vestige  of  it  present,  which,  if  the  quan- 
tity of  gas  was  increased,  so  that  the  observer 
could  look  towards  immense  thicknesses,  would  be 
increased  until  the  emission  for  all  wave  lengths 
would  finally  approach  the  intensity  of  a  perfect  radi- 
ator. This  view  is  supported  by  the  circumstance 
that,  if  the  pressure  upon  the  emitting  gas  is  in- 
creased to  several  atmospheres,  the  spectrum  lines 
widen  out,  till  at  length  there  is,  for  some  distance 
from  the  lines,  a  perceptible  continuous  background. 
Whether  or  not,  then,  it  be  true  that  gases  under 
7  71 


THE  SUN 

less  than  atmospheric  pressure  would  give  continu- 
ous spectra  if  in  great  depth,  it  is  certainly  highly 
probable  that  such  gases  would  do  so  if  more  and 
more  compressed  with  the  increasing  thickness.  Re- 
gions of  strong  emission  are  regions  of  strong  absorp- 
tion by  Kirchhoff  s  law,  so  that  in  the  case  of  a  thick 
gas,  as  just  proposed,  it  would  be  only  the  front 
layers  which  would  give  rise  to  the  lines  or  bands  of 
high  selective  absorption,  while  the  deep-lying  layers 
would  be  those  which  would  produce  the  continuous 
spectrum.  If  the  gas  is  not  of  uniform  temperature, 
but  grows  hotter  with  increasing  thickness,  it  is  easy 
to  see  that  the  continuous  spectrum  might  exceed 
the  line  spectrum  in  its  intensity,  so  that  the  really 
bright  lines  would  appear  dark  by  contrast  with  the 
background.  As  is  well  known,  the  solar  spectrum 
has  the  character  of  a  continuous  bright  ground 
crossed  by  darker  lines,  and  evidence  will  be  pre- 
sented later  which  indicates  that  it  is  indeed  to  be 
regarded  as  a  gaseous  spectrum  of  the  kind  just 
described. 

PYRHELIOMETRY 

In  the  year  1838  Pouillet  devised  the  instru- 
ment which  he  called  the  pyrheliometer,  shown  in 
Fig.  18,  and  used  it  for  measuring  the  intensity  of 
the  sun's  radiation.  A  flat  silver-plated  vessel  ab, 
blackened  with  lampblack  on  its  upper  surface,  is 
filled  with  water,  and  contains  also  the  bulb  of  the 
thermometer  d.  The  instrument  is  held  in  the 

72 


SOLAR  INVESTIGATION 

clamp  c,  and  pointed  towards  the  sun  as  indicated 
when  the  shadow  of  the  box  ab  falls  centrally  on  the 
plate  ee.  By  rotating  the  whole  apparatus  in  the 
clamp  c,  the  water  can,  in  effect,  be  stirred  to  equalize 
its  temperature.  To  observe  the 
intensity  of  the  solar  radiation 
the  instrument  is  first  shaded, 
and  the  change  of  temperature 
occurring  in  a  certain  time,  as, 
for  instance,  five  minutes,  is 
noted.  Then  the  screen  is  re- 
moved/ and  the  observer  notes 
the  change  of  temperature  due 
to  the  sun's  heating  in  the  same 
time.  Finally  the  shade  obser- 
vation is  repeated.  Correcting 
the  average  rate  of  rise  of  tem- 
perature per  minute  during  the 
sun  exposure  by  the  average 
rate  of  cooling,  shown  by  the 
shade  readings,  the  result  gives 

the  rise  of  temperature  per  min- 

V.  ,  FIG.  is. 

ute  of  a  mass  of  water  and  cop- 
per, of  known  heat  capacity,  due  to  the  sun's  rays 
shining  at  right  angles  and  absorbed  on  the  known 
area  of  the  top  of  the  box.  A  correction  of  about 
2.5  per  cent  must  be  added  on  account  of  loss  by 
reflection  from  the  lampblack. 

Pouillet  observed  the  intensity  of  the  sun's  rays 
with  this  instrument  at  different  hours  of  the  day. 

73 


THE  SUN 

The  atmosphere  weakens  the  sun  rays  by  the  diffuse 
reflection  of  its  molecules  and  dust  particles.  This 
effect  is  more  and  more  apparent  as  the  sun  nears 
the  horizon.  The  atmosphere  extends  upwards  for 
a  great  distance,  but  becomes  less  and  less  dense, 
so  that  at  one  hundred  miles  elevation  what  re- 
mains above  is  negligible,  so  far  as  cutting  off  the 
sun's  rays  is  concerned.  Hence,  we  may  regard  the 
effective  part  of  the  atmosphere  as  a  layer  whose 
thickness  is  very  small  compared  to  the  earth's  ra- 
dius; and  so,  whenever  the  sun  is  15°  or  more  above 
the  horizon,  the  length  of  path  of  its  rays  in  air  is  in 
proportion  to  the  length  of  the  path  when  the  sun  is 
in  the  zenith  simply  as  the  secant  of  the  zenith  dis- 
tance at  the  time  of  the  observation. 

Bouguer  and  Lambert  had  shown  independently, 
in  the  year  1760,  that  when  a  ray  traverses  a  homo- 
geneous transparent  medium,  the  intensity,  E,  after 
traversing  any  given  thickness,  t,  of  the  medium  is 
given  by  the  following  formula,  in  which  E0  is  the 
original  intensity,  and  a  is  a  constant  which  repre- 
sents the  proportion  transmitted  by  unit  thickness: 

E  =  E0af. 

Pouillet  applied  Bouguers  formula  to  his  observa- 
tions, taking  unit  thickness  as  that  traversed  by 
rays  when  the  sun  is  in  the  zenith,  so  that  if  z  is  the 
zenith  distance,  the  formula  becomes: 

E  =  E0asecantz. 

He  computed  the  value  E0,  which  is  the  intensity  of 
the  sun's  radiation  outside  the  atmosphere,  and,  re- 

74 


SOLAR   INVESTIGATION 

ducing  to  mean  distance  of  the  sun1,  obtained  E0 
=  1.76  calories  per  square  centimeter  per  minute. 
This  value  Radau,  and,  also,  Langley  afterwards 
showed  must  be  below  the  true  value  of  the  "  solar 
constant  of  radiation"  because  Pouillet  made  no 
spectrum  observations,  and  it  is  necessary  to  do  so  on 
account  of  the  unequal  losses  suffered  by  rays  of  dif- 
ferent wave  lengths  in  passing  through  the  air. 

Pouillet's  pyrheliometer  was  improved  by  Tyndall, 
who  substituted  an  iron  box  containing  mercury  in 
place  of  the  copper  box  containing  water.  In  re- 
cent years  Tyndall' s  design  has  been  improved  at 
the  Smithsonian  Institution.  First,  a  copper  box 
filled  with  mercury  was  employed;  then  a  copper 
disk  with  a  hole  drilled  radially  to  contain  the  cylin- 
drical bulb  of  a  thermometer  with,  also,  a  little  mer- 
cury surrounding  it  to  make  good  heat  connection; 
now  (1910),  the  Institution  uses  a  blackened  silver 
disk  (shown  in  section  at  a  in  Fig.  19)  with  a  radial 
hole  lined  by  a  thin  steel  thimble.  In  this  is  inserted 
in  mercury  a  cylindrical-bulb  thermometer,  6,  bent 
at  right  angles  so  as  to  point  towards  the  sun  when  in 
use.  The  disk  is  enclosed  in  a  brass-walled,  black- 
ened chamber,  c,  and  this  is  protected  from  changes 
of  temperature  by  a  wooden  wall,  d,  outside.  The 
sun's  rays  are  admitted  through  a  tube,  e  (shown 
partly  in  section),  which  contains  diaphragms,  ///, 

irThe  sun's  radiation  varies  in  its  intensity  inversely  as  the 
square  of  the  sun's  distance.  Hence  the  earth  receives  on  this 
account  nearly  7  per  cent  more  solar  radiation  in  January  than  in 
July. 

75 


THE  SUN 


FIG.  19. — SILVER  DISK  PYRHELIOMETER. 

76 


SOLAR  INVESTIGATION 


to  prevent  air  currents  from  reaching  the  silver  disk. 
An  equatorial  mounting  enables  the  observer  to 
point  the  instrument  towards  the  sun.  Several  in- 
struments of  the  type  shown  in  Fig.  19  have  been 

constructed  and  com- 
pared with  those  at  the 
Institution,  and  sent  to 


FIG.  20 — ANGSTROM'S  PYRHE- 


LIOMETER. 


FIG.  21. 


different  solar  observers  abroad  to  convey  to  them 
exactly  the  scale  of  measurements  employed  here. 
In  1896  K.  Angstrom  devised  his  electrical  com- 
pensation pyrheliometer,  which  has  been  used  very 
extensively.  Fig.  20  gives  a  general  view  of  the 
instrument  and  Fig.  21  an  enlarged  detail  view  of 
the  interior.  It  consists  of  two  thin  strips  of  man- 
ganin,  U  U,  of  measured  area,  which  are  blackened  on 
the  front  surfaces,  and  have  fixed  to  the  rear  of  each 
a  thermoelectric  junction  for  determining  their  tem- 
peratures. The  binding  posts,  Kx  K2,  communicate 
respectively  to  the  strips  and  the  thermal  junc- 
tions. A  measured  current  of  electricity  is  passed 
through  one  strip,  while  the  other  is  exposed  to  the 
sun,  and  when  a  galvanometer  connected  with  the 

77 


THE  SUN 

thermal  junctions  indicates  equality  of  temperature 
it  is  assumed  that  the  known  amount  of  heat  in- 
troduced by  the  electrical  current  is  equal  to  that 
absorbed  from  the  sun's  rays.  By  reversing  the 
screen,  W,  and  the  commutator,  C,  the  two  strips 
are  heated  alternately  by  the  sun  and  by  electricity, 
and  the  mean  result  is  employed.  After  applying  a 
correction  for  loss  by  reflection,  the  results  are  com- 
puted in  terms  of  calories  per  square  centimeter 
per  minute.  The  instrument  is  inclosed  in  a  dia- 
phragmed  tube  R,  and  is  mounted  on  an  alt-azimuth 
stand  provided  with  the  screws,  Si  S2,  for  following 
the  sun.  A  thermometer,  T,  indicates  the  tempera- 
ture of  tihe  strips. 

In  both  forms  of  pyrheliometers  described  above, 
if  used  as  standard  instruments,  a  correction 
must  be  determined  and  applied  to  allow  for  the 
radiation  reflected.  Besides  this,  there  is  another 
source  of  loss,  arising  from  the  fact  that  part  of  the' 
heat  produced  by  the  absorption  of  solar  radiation 
in  lampblack  is  carried  off  by  the  air,  and  by  re- 
radiation  of  great  wave  length,  and  this  part  does 
not  produce  any  effect  on  the  thermometer  or  ther- 
moelectric junction. 

To  avoid  these  sources  of  errpr  other  forms  of 
pyrheliometers  have  been  devised  in  which  the  rays 
are  absorbed  within  a  hollow  cylindrical  blackened 
chamber.  Such  a  chamber,  as  stated  a  few  pages 
above,  is  practically  a  perfect  radiator,  and  hence 
is  a  perfect  absorber,  so  that  no  correction  for  rays 


SOLAR  INVESTIGATION 


reflected  is  needed.  The 
rays  are  principally  ab- 
sorbed at  the  rear  end, 
and,  as  the  tube  is  deep, 
the  heat  tending  to  es- 
cape will  be  absorbed 
somewhere  on  the  side 
walls.  Two  means  of 
using  the  hollow  cham- 
ber have  been  employed, 
the  first  about  1894,  by 
W.  A.  Michelson,  the 
second,  1905  to  1910,  by 
the  writer.  Michelson 
surrounds  the  chamber 
by  melting  ice  and  water, 
and  determines  the  heat 
introduced  by  measuring 
the  contraction  of  the 
ice  as  it  melts. 

In  the  form  devised  by 
the  writer,  as  shown  in 
Fig.  22,  a  measured 
stream  of  water,  enter- 
ing at  E  and  emerging 
at  F,  flows  continually 
in  a  spiral  channel  round 
the  walls  of  the  blackened 
chamber,  A  A,  carrying 
off  the  heat  as  fast  as 


79 


THE  SUN 

formed.  The  rise  of  temperature  of  the  stream  of 
water  due  to  the  solar  heating  (admitted  through  the 
vestibule,  B  B,  and  the  measured  diaphragm,  C)  is  de- 
termined by  a  differential  electrical  thermometer  com- 
posed of  four  fine  platinum  wires  wound  longitudi- 
nally on  ivory  spirals.  These  wires  are  bathed  by  the 
stream  of  water  which  follows  the  spiral  channels  of  the 
ivory.  Two  coils  are  situated  at  Dx,  in  the  entering 
stream  of  water,  and  two  at  D2,  after  its  passage 
through  the  walls  of  the  chamber.  The  four  are  joined 
to  form  a  Wheatstone's  bridge,  and  their  indications 
are  read  by  a  sensitive  galvanometer.  The  pyrheliom- 
eter  is  protected  from  outside  temperature  changes 
by  the  Dewar  vacuum  flask,  K  K.  In  order  to  test 
the  accuracy  of  the  instrument  two  coils  of  man- 
ganin  wire,  G  and  H,  are  placed  within  the  chamber 
near  its  rear,  and  a  known  quantity  of  heat  may  be 
produced  there  in  either  coil  by  the  passage  of 
a  measured  current  of  electricity.  This  heat  is  then 
measured  just  as  if  it  were  from  the  sun,  and  if  all 
that  is  introduced  is  found,  it  may  be  supposed  that 
the  instrument  is  a  correct  recorder  of  solar  radia- 
tion, especially  as  the  coil  G  is  very  unfavorably  sit- 
uated for  giving  up  its  heat  to  the  walls. 

Two  such  water-flow  pyrheliometers  of  different 
dimensions  were  tested  at  Washington  in  1910  and 
gave  closely  agreeing  results  on  solar  radiation,  be- 
sides recovering  almost  completely  the  electrically 
developed  heat  used  as  a  test.  These  water-flow 
pyrheliometers  are  used  as  standards,  and  the  read- 

80 


SOLAR  INVESTIGATION 


ings  of  the  silver-disk  pyrheliometers  are  reduced 
to  the  scale  they  give.  The  water-flow  pyrheliometer, 
when  in  use,  is  mounted  equatorially  and  driven  by 
clockwork  to  follow  the  sun.  It  is  alternately 
shaded  and  exposed  to  solar  radiation. 

BOLOMETRY 

For  measuring  the  intensity  of  the  rays  in  the 
solar  spectrum,  the  instrument  most  used  is  the 
bolometer,  a  delicate  electrical  thermometer,  in- 
vented by  Langley  about  1880.  As  now  construct- 
ed, it  comprises  two  exactly 
similar,  narrow,  blackened, 
platinum  strips  hardly  as 
wide  as  hairs,  ten  times 
thinner  than  they  are  wide, 
and  about  half  an  inch 
long.  Referring  to  Fig.  23, 
such  strips,  a,  6,  having  an 
electrical  resistance  of  about 
four  ohms  each,  are  joined,  as  shown,  to  two  coils,  c,  d, 
of  manganin  wire,  each  of  about  20  ohms  resistance, 
forming  with  the  two  strips  a  Wheatstone's  bridge. 
A  variable  resistance,  e,  of  several  thousand  ohms  is 
shunted  around  one  coil  and  serves  to  bring  the 
whole  to  an  electrical  balance.  Sometimes  a  small 
resistance  of  copper,  /,  is  included  in  one  arm  of  the 
Wheatstone's  bridge  to  prevent  its  unbalancement 
as  the  surrounding  temperature  changes.  A  cur- 
rent of  about  0.1  ampere  from  a  storage  battery  of 

81 


— IjJ — O-wVWV-O 


FIG.  23. 


THE  SUN 

several  cells,  in  parallel,  flows  constantly  through 
the  bridge,  and  the  adjustment  is  observed  by  a 
highly  sensitive  galvanometer,  g.  If  the  radiation 
is  caused  to  fall  on  one  of  the  bolometer  strips,  its  re- 
sistance increases,  and  there  results  a  deflection  of 
the  galvanometer  proportional  to  the  heat  produced 
by  the  radiation.  The  record  of  the  galvanometer  is 
kept  automatically  on  the  photographic  plate  which 
is  moved  vertically  by  the  clockwork  at  the  same 
time  that  the  spectrum  is  moved  across  the  bolom- 
eter strip,  so  that  rising  and  falling  temperatures 
of  the  strip,  due  to  changes  of  intensity  of  the  spec- 
trum, are  indicated  by  higher  and  lower  parts  of  the 
curve,  photographically  traced  by  the  little  spot  of 
sunlight  reflected  by  the  tiny  mirror  of  the  galvanom- 
eter needle.  Fig.  24  gives  a  pair  of  such  energy 
curves  or  bolographs  of  the  solar  spectrum.  Some 
of  the  principal  Fraunhofer  lines  give  great  depres- 
sions of  the  curve,  and  are  indicated  on  the  margin 
of  the  figure.  At  the  points  marked  *  *  a  shutter 
was  introduced  in  front  of  the  slit  of  the  spectro- 
scope to  give  the  zero  of  radiation.  At  the  points 
marked  f  f  diaphragms  were  introduced  to  diminish 
the  intensity  of  the  spectrum,  so  that  the  photo- 
graphic trace  would  not  run  off  the  plate.  The  scale 
of  the  intensity  as  thus  altered  is  indicated  on  the 
margin. 

In  Chapters  III  and  VII  are  giveri  the  applica- 
tion of  the  bolometer  for  the  determination  of  the 
"solar  constant  of  radiation,"  the  transparency  of 

82 


SOLAR   INVESTIGATION 


83 


THE  SUN 

the  atmosphere  for  rays  of  different  wave  lengths,  the 
investigation  of  the  comparative  brightness  of  dif- 
ferent parts  of  the  solar  image,  and  the  determina- 
tion of  the  temperature  of  the  sun.  The  astonishing- 
sensitiveness  of  the  bolometer  may  be  understood 
when  it  is  said  that,  in  ordinary  use,  changes  of 

temperature  of  less  than  77^7:7^  of  a  degree  C.  are 

100,000 

measured,  and  by  special  installation  this  sensitive- 
ness may  be  increased  1000  fold.  The  still  more 
astonishing  sensitiveness  of  the  eye  is  indicated  by 
the  fact  that  we  receive  enough  light  through  the 
pupil  of  the  eye  from  a  star  of  the  sixth  magnitude 
to  see  it,  though  with  the  most  sensitive  bolometer 
it  would  require  a  mirror  perhaps  ten  feet  in  diameter 
to  concentrate  enough  rays  from  such  a  star  to  make 
its  heating  observable.  This  is  the  more  striking 
because  the  eye  is  affected  by  only  a  short  range  of 
spectral  colors,  while  the  bolometer  measures  the 
total  radiation  of  all  wave  lengths. 


CHAPTER   III 

THE    PHOTOSPHERE 

Telescopic  View. — The  Photospheric  Spectrum. — Rowland's  Spec- 
trum Tables. — Chemical  Elements  Found  and  Not  Found. — 
Corrections  to  Rowland's  Wave  Lengths. — Levels. — Pressures. — 
Convection  Currents. — Limb  Spectra. — Variation  of  the  Sun's 
Brightness. — Solar  Temperatures. — Spectroheliography. — Solar 
Rotation. 

As  viewed  through  the  telescope,  or  photographed, 
the  radiating  surface  of  the  sun,  called  the  "  photo- 
sphere, "  presents  a  brilliant  disk  covered  by  indis- 
tinct mottlings  sometimes  spoken  of  as  the  "  rice- 
grain-structure."  Objects  much  less  than  a  second  of 
arc  or  400  miles  in  diameter,  cannot  be  well  seen  on 
the  sun,  so  that  these  "  rice-grains, "  which  appear 
according  to  different  authors  from  100  to  500  miles 
in  diameter,  are  really  large  areas.  Some  authors 
speak  of  the  bright  areas  of  this  mottled  appearance 
as  " granulations, "  and  the  darker  parts  as  "pores." 
Generally  a  few  very  dark  patches  called  "sun 
spots"  may  be  seen,  and  around  them,  if  they  hap- 
pen to  be  observed  near  the  edge  or  "limb"  of 
the  sun,  are  found  very  bright  areas  called  "faculae. " 
The  facula?  are  seldom  seen  very  much  more  than  a 
quarter  radius  within  the  limb.  Photography  reveals 
at  once,  what  the  eye  recognizes  less  easily,  that  the 

85 


THE  SUN 

photosphere  falls  off  in  brightness  towards  the  sun's 
limb.  A  photograph  well  exposed  at  the  center  will 
be  very  weak  at  the  limb.  Plate  III  shows  this 
clearly,  and  also  exhibits  .the  rice-grain  structure, 
sun  spots,  and  faculsc.  Sun  spots  march  nearly  reg- 
ularly across  the  sun's  disk  in  about  13. 6 1  days,  and 
appear  after  an  equally  long  absence,  which  indicates 
that  the  sun  rotates  upon  its  axis. 

THE  PHOTOSPHERIC  SPECTRUM 

The  spectrum  of  the  sun's  photosphere  is  a  con- 
tinuous bright  background  of  color  crossed  by  dark 
lines  and  bands.  Newton  recognized  seven  colors  in 
the  spectrum,  comprising  violet,  indigo,  blue,  green, 
yellow,  orange  and  red,  but  these  blend  into  one 
another  by  perfectly  imperceptible  gradations  of  in- 
numerable hues.  By  photography  and  by  the  bolom- 
eter, the  solar  spectrum  has  been  followed  beyond 
the  violet  end  as  seen  by  the  eye  (which  occurs 
about  wave  length  0.38A*),  as  far  as  wave  length  0.29/*. 
Here  the  rays  are  almost  wholly  cut  off  by  losses  in 
the  earth's  atmosphere  and  in  the  sun's  outer  envel- 
opes. Beyond  the  red,  which  may  be  observed  with 
the  eye  to  wave  length  0.80/*,  Abney  has  photo- 
graphed, by  the  aid  of  specially  dyed  plates,  to  wave 
length  l.l/*,  and  with  the  bolometer  the  solar  spec- 
trum has  been  measured  at  the  Smithsonian  Astro- 
physical  Observatory  as  far  as  wave  length  5.3/*. 

irThe  earth  is  meanwhile  advancing,  so  that  this  is  not  the  half 
period  of  the  sun's  sidereal  rotation. 

86 


DIRECT  SOLAR  PHOTOGRAPH.     (Ellerman.^ 
1908,  April  30.     G.  M.  T.  2  h  30  m.     P.  S.  T.  6  h  30  m  A.  M. 


THE  PHOTOSPHERE 

Probably  sun  rays  might  be  recogr/zed  with  the 
bolometer  at  intervals  as  far  as  20/S  but  beyond 
this  they  would  probably  be  practically  all  cut  off 
by  losses  in  the  earth's  atmosphere. 

The  dark  lines  and  bands  of  the  solar  spectrum, 
named  from  their  discoverer  "Fraunhofer  lines," 
have  two  different  sources.  A  considerable  number 
of  lines,  notably  in  the  red  and  infra-red  regions  of  the 
spectrum,  are  caused  by  the  absorption  of  gases  and 
vapors  in  the  earth's  atmosphere.  The  chief  of 
these  terrestrial  absorbents  are  oxygen,  water  vapor, 
and  carbonic-acid  gas.  By  far  the  greater  number 
of  the  Fraunhofer  lines,  however,  are  formed  by  the 
absorption  of  solar  rays  by  gases  in  and  about  the 
sun  itself;  notably  by  iron,  nickel,  calcium,  titanium, 
cobalt,  chromium,  manganese,  carbon,  vanadium, 
sodium,  magnesium,  and  hydrogen.  The  existence  of 
these  elements  and  many  others  in  the  sun  is  proved 
by  the  occurrence  in  the  solar  spectrum  of  dark  lines, 
occupying  the  same  relative  positions  as  to  wave 
length,  and  generally  of  nearly  the  same  relative  in- 
tensity, that  the  characteristic  bright  lines  of  these 
elements  occupy  in  their  spectra  as  produced  in  the 
laboratory.  As  shown  by  Kirchhoff  and  Bunsen  in 
1859,  dark  lines  are  produced  in  a  bright  continuous 
spectrum  by  interposing  cooler  vapors  or  gases  be- 
tween the  source  of  light  and  the  spectroscope,  and 
these  lines  occupy  the  same  positions  that  the  bright 
lines  of  the  vapors  or  gases  would  occupy  if  the  latter 
were  themselves  the  sole  sources  of  light,  Conform- 
8  87 


THE  SUN 

ably  to  this  discovery  it  will  be  shown  in  a  later  chap- 
ter that  the  spectrum  of  the  outermost  solar  layer, 
called  the  "  chromosphere, "  when  seen  alone  at  solar 
eclipses,  is  a  bright  line  spectrum  vvhich  is  almost  the 
exact  reversal  of  the  photospheric  spectrum.  The 
layer  in  which  the  dark  lines  have  their  rise  is  accord- 
ingly called  "the  reversing  layer." 

As  any  gases  between  the  observer  and  the  sun  may 
produce  dark  absorption  lines  in  this  way,  it  is  not 
at  first  apparent  how  to  distinguish  between  terres- 
trial and  solar  gases.  There  are  two  ways  of  testing 
whether  a  given  Fraunhofer  line  is  solar  or  atmos- 
pheric. The  first  is  by  observing  its  intensity  rela- 
tive to  other  lines  at  high  and  low  elevation  of  the 
sun  above  the  horizon.  Atmospheric,  or  as 'they  are 
called,  "telluric,"  lines  will  generally  be  strengthened 
at  low  sun,  because  the  layer  of  air  traversed  will  then 
be  greater.  A  second  and  better  method  of  discrim- 
ination consists  in  forming  an  image  of  the  sun  and 
causing  rays  from  its  east  and  west  limbs  to  be  re- 
flected together  simultaneously  into  the  slit  of  the 
spectroscope,  so  as  to  give  rise  to  two  superposed 
solar  spectra,  one  of  light  from  the  east  limb,  the 
other  from  the  west.  Telluric  lines  will  occupy  the 
same  position  in  the  two  spectra,  but  solar  lines  will 
be  shifted  with  reference  to  one  another  owing 
to  the  rotation  of  the  sun,  which  produces  a  very 
notable  Doppler  effect.  This  is  shown  in  Plate 
IV,  Fig.  2,  -which  includes  the  oxygen  band  B  and 
some  solar  lines  in  its  vicinity  as  photographed 


THE   PHOTOSPHERE 

at  Mount  Wilson  by  St.  John  under  remarkably 
fine  conditions. 

ROWLAND'S  SPECTRUM  TABLES 

The  solar  spectrum  has  been  photographed  at  great 
dispersion  by  numerous  observers,  but  most  notably 
by  Rowland.  He  published  about  1895,  in  the  early 
volumes  of  the  Astrophysical  Journal,  his  great  "  Pre- 
liminary Table  of  Solar  Spectrum  Wave  Lengths," 
which  still  forms  the  basis  for  solar  and  stellar  re- 
searches. Rowland  states  in  his  introduction  that 
he  photographed  the  arc  spectrum  of  all  the  then 
known  elements  except  gallium  in  connection  with 
the  solar  spectrum,  but  that  the  work  of  identifica- 
tion of  lines  in  the  solar  spectrum  with  the  arc  lines 
would  be  a  further  labor  of  years.  This  work  of 
identification  has  never  yet  been  completed,  nor  has 
a  correspondingly  full  comparison  of  the  solar  spec- 
trum with  the  spark  spectra  of  the  elements  been 
attempted.  In  Rowland's  " Preliminary  Table"  there 
are  about  14,000  lines  recorded.  Their  wave  lengths 
are  given  to  seven  places  of  figures,  that  is,  to  thou- 
sandths of  an  Angstrom.  For  each  line  is  given  its 
intensity.  The  intensities  go  from  1,  a  line  just 
clearly  visible  on  Rowland's  spectrum  map,  up  to 
1000  for  the  strong  calcium  lines  H  and  K.  Below 
1,  the  intensities  go  down  to  0000,  indicating  lines 
more  and  more  difficult  to  see. 

The  great  lines  of  the  solar  spectrum,  named  long 
ago  for  the  letters  of  the  alphabet,  are  as  follows : 

89 


THE  SUN 


TABLE  III. — Principal  solar  spectrum  lines. 


Line 

A 

7593.842 
Oxygen3 

a 

7184.57 
Waters 

B 

6869.970 
Oxygen3 

C  (Ha) 

6562  .  835 
Hydrogen 

D2 

5889  .  975 
Sodium 

E 

5269.551 
Iron 

Corrected 
wave  length1  

Element 

Line  
Corrected 
wave  length1  
Element 

b 

5183  .  620 
Magnesium 

F(H/3) 

4861.350 
Hydrogen 

G(Hy) 

4340.471 
Hydrogen 

H 

3968.491 
Calcium 

K 

3933  .  680 
Calcium 

1  According  to  the  table  of  corrections  below. 

2  Edge  of  the  head  of  A. 
8  Terrestrial  lines. 

About  one- third  of  the  14,000  solar  lines  were  iden- 
tified by  Rowland  and  ascribed  by  him  to  various 
chemical  elements.  In  a  good  many  cases  a  line 
is  attributed  to  several  elements  at  once.  In  such 
cases  the  coincidence  with  them  all- is  probably  not 
generally  exact,  but  only  so  close  that,  even  with 
Rowland's  very  high  dispersion,  the  several  lines 
overlap.  Investigations  with  much  higher  dispersion 
on  bright  line  spectra  indicate  that  in  many  cases 
apparently  single  lines  of  single  elements  are  really 
resolvable  into  groups.  But  perhaps  even  such  very 
high  resolving  powers  would  generally  fail  to  separate 
the  blended  lines  of  Rowland's  table,  because,  owing 
to  pressure  or  other  conditions,  the  several  lines 
involved  are  so  much  widened  as  to  overlap.  For 
many  years  Lockyer  maintained  plausibly  that  the 
elements  had  common  constituents  which  gave  rise 
to  common  lines  in  the  spectrum,  but  this  so-called 
" basic  line"  hypothesis  is  not  now  generally  held. 
The  following  summary  of  Rowland's  identifications 

90 


THE   PHOTOSPHERE 


is   taken   from   Young's   "The   Sun,"    with   slight 
changes : 

Chemical  Elements  Found  and  not  Found  in  the 
Sun. — The  first  columns  of  the  following  table  give 
the  chemical  elements  found  by  Rowland  to  exist 
in  the  sun  arranged  according  to  the  intensity  of  their 
solar  lines,  and  with  their  atomic  weights  annexed. 
The  last  columns  give  them  arranged  in  the  order  of 
the  number  of  their  solar  lines,  and  with  the  numbers 
occasionally  annexed.  The  sign  f  indicates  that 
the  element  has  not  been  identified  in  eclipse  chro- 
mospheric  spectra. 


TABLE  IV. — Chemical  elements  found  in  the  sun. 


1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 
17. 
18. 
19. 
20. 
21. 
22. 
23. 


Calcium  (40.09) 
Iron  (55.85) 
Hydrogen  (1.008) 
Sodium  (23.00) 
Nickel  (58.68) 
Magnesium  (24.32) 
Cobalt  (58.97) 
Silicon  (28.3) 
Aluminum  (27.1) 
Titanium  (48.1) 
Chromium  (52.0) 
Strontium  (87.62) 
Manganese  (54.93) 
Vanadium  (51.2) 
Barium  (137.37) 
Carbon  (12.00) 
Scandium  (44.1) 
Yttrium  '(89.0) 
Zirconium  (90.6) 
fMolybdenum  (96.0) 
Lanthanum  (139.0) 
fNiobium  (93.5) 
fPalladium  (106.7) 


Iron  (2000  or  more) 
Nickel 
Titanium 
Manganese 
Chromium 
Cobalt 

Carbon  (200  or  more) 
Vanadium 
Zirconium 
Cerium 

Calcium  (75  or  more) 
Neodymium 
Scandium 
Lanthanum 
Yttrium 
Niobium 
Molybdenum 
Palladium 

Magnesium  (20  or  more) 
Sodium  (11) 
Silicon 
Hydrogen 
Strontium 
91 


THE  SUN 

24.  tNeodymium  (.144.3)  Barium 

25.  fCopper  (63.57)  Aluminum  (4) 

26.  Zinc  (65.37)  Cadmium 

27.  Cadmium  (112.40)  Rhodium 

28.  Cerium  (140.25)  Erbium 

29.  tGlucinum  (9.1)  Zinc 

30.  tGermanium  (72.5)  Copper  (2) 

31.  fRhodium  (102.9)  Silver 

32.  Silver  (107.88)  Glucinum 

33.  Tin  (119.0)  Germanium 

34.  Lead  (207.10)  Tin 

35.  Erbium  (167.4)  Lead  (1) 

36.  "fPotassium  (39.10)  Potassium 

Besides  these  thirty-six  elements,  thus  arranged,  it 
has  been  found  that  helium  (4.0)  and  gallium  (69.9) 
certainly  show  solar  lines,  although  helium  lines  are 
hide-and-seek  things,  and  for  some  reason  only  occa- 
sionally appear  as  dark  lines  in  the  solar  spectrum. 
There  also  appear  very  faint  dark  solar  lines,  nearly 
or  exactly  corresponding  in  their  position  to  some  of 
the  strongest  arc  lines  of: 

TABLE  V. — Chemical  elements  doubtfully  occurring  in  the  sun. 

Ruthenium    (101.7),    Indium          (114.8),    Tantalum  (181.0), 

Tungsten       (184.0),     Osmium        (190.9),     Iridium  (193.1), 

Platinum        (195.0),     Mercury        (200.0),     Thallium  (204.0), 

Bismuth         (208.0),    Thorium       (232.42),  Uranium  (238.5). 
The  mean  atomic  weight  of  these  elements  is  186 . 95. 

The  lines  of  the  important  elements  of  the  halogen 
group,  Fluorine,  Chlorine,  Bromine,  Iodine;  those 
of  the  oxygen  group,  Oxygen,1  Sulphur,  Selenium,  and 

1  Since  this  was  written  St.  John  has  found  that  a  triplet  of  faint 
lines  attributed  to  oxygen  occurring  beyond  A  in  the  extreme  red, 
shows  relative  displacements  at  the  sun's  limbs.  Hence,  we  must 
probably  admit  free  oxygen  as  giving  a  solar  spectrum.  Combined 
oxygen  and  combined  nitrogen  give  solar  band  spectra. 

92 


THE   PHOTOSPHERE 

Tellurium;  those  of  the  nitrogen  group,  Nitrogen, 
Phosphorus,  Arsenic,  and  Antimony  (Bismuth  doubt- 
ful) do  not  appear  to  have  been  found  in  the  photo- 
spheric  spectrum,  or  in  the  spectrum  of  the  chromo- 
sphere. This  singular  omission  comprises  nearly  all 
of  the  prominent  " negative"  elements,  and  Boron, 
another  of  them,  is  also  absent  from  the  solar  spec- 
trum. Further  remarks  on  this  subject  will  be  made 
later. 

There  is  considerable  interest  attaching  to  the 
relations  of  atomic  weight  of  the  elements  and  the 
intensity  of  their  solar  lines.  Taking  the  thirty-six  ele- 
ments of  the  intensity  table  in  order,  in  four  groups  of 
nine  each,  the  average  atomic  weights  are  as  follows: 

Elements    1-9,    35.26;    elements  10-18,    64.04; 
elements  19-27,  101.27;    elements  28-36,  107.25. 

In  the  last  group,  as  thus  divided,  occur  glucinum 
(9.1)  and  potassium  (39.10).  The  former  has  two, 
and  the  latter  one  identified  line,  and,  as  these  lines 
are  also  very  weak,  it  is  not  impossible  that  these  two 
elements  may  by  future  investigation  fall  out  of  their 
strange  company.1  If  so,  the  mean  atomic  weight  of 
the  remaining  seven  elements  would  be  131.00.  In 
Group  II  of  this  arrangement  appears  carbon  (12.00), 
but,  judging  from  Kayser's  "  Handbuch,"  the  solar 
"  carbon  "  lines  belong  to  carbon  compounds  of  high 
molecular  weights.  Hardly  less  interesting  than  the 

1  Kayser  and  Runge  question  the  existence  of  potassium  lines  in 
the  photospheric  spectrum. 

93 


THE  SUN 

classification  just  given  is  the  further  fact  that  most 
of  the  elements  of  the  platinum  group,  and  some  other 
elements  of  very  high  atomic  weight  found  commonly 
on  the  earth,  are  only  doubtfully  recognized  in  the 
sun,  although  they  give  strong  lines  in  the  arc.  The 
full  significance  of  these  relations  will  be  further  dis- 
cussed in  Chapter  VI,  but  it  may  be  said  here  that 
the  explanation  of  the  decrease  of  intensities  with  in- 
creasing atomic  weights  seems  to  depend  on  the  depth 
of  these  gases  below  the  sun's  surface.  We  may  sup- 
pose that  the  interesting  elements  radium  and  ura- 
nium might  not  produce  lines  in  the  solar  spectrum, 
even  if  these  elements  exist  in  the  sun,  because  of 
their  high  atomic  weights. 

The  element  oxygen  undoubtedly  exists  in  the  sun 
because  the  flutings  of  titanium  oxide  are  very  prom- 
inent in  sun-spot  spectra.  It  might  be  anticipated 
that  the  well-known  oxygen  lines  themselves  would 
be  found  in  the  photospheric  spectrum  if  it  were  not 
that  the  earth's  atmosphere  itself  contains  so  much 
oxygen  as  to  produce  such  intense  oxygen  lines  that 
solar  effects  are  unrecognizable.  However,  photo- 
graphs of  the  spectra  of  the  two  opposite  limbs  of 
the  sun  show  the  negative,  for  in  these  spectra  all 
solar  lines  are  displaced  by  Doppler  effects,  but  the 
well-known  oxygen  lines  show  none.  Nitrogen,  also 
found  plentifully  in  the  earth's  atmosphere,  behaves 
similarly.  It  is  a  peculiar  feature  of  the  solar  spec- 
trum that  very  few  of  the  so-called  negative,  or  non- 
metallic,  elements  are  recognized  from  it.  Thus,  the 

94 


THE  PHOTOSPHERE 

important  halogen  group  of  elements,  which  includes 
such  common  elements  as  chlorine  and  bromine,  is 
unrecognized.  So  also  with  the  important  element 
sulphur.  These  omissions  are  very  remarkable  and 
not  yet,  I  think,  well  understood. 

However,  it  is  found  frequently  in  the  laboratory 
that  the  spectrum  of  a  mixture  or  compound  of  two 
elements  is  apt  to  show  one  of  them  predominatingly, 
or  even  alone.  Especially  does  a  metal  thus  often  ex- 
clude a  nonmetal.  But  yet  oxygen  and  helium,  which, 
although  existing  in  the  sun,  are  of  slight  effect  in  the 
solar  spectrum,  are  very  prominently  in  evidence  in 
the  spectra  of  many  of  the  stars.  Since  oxygen  is 
certainly  present  in  sun  spots  as  an  oxide,  and  nitro- 
gen as  cyanogen,  though  they  do  not  give  their  char- 
acteristic lines  as  elements,1  the  other  elements  just 
mentioned  may  also  be  present  in  the  sun  without 
giving  their  spectral  lines. 

Some  of  the  " unknown"  lines  have  now  been  as- 
signed to  their  appropriate  elements,  but  more  than 
half  of  Rowland's  lines  are  still  unidentified.  A  large 
number  of  these  are,  however,  very  weak.  It  is 
probable  that  within  the  next  decade  many  of  them 
will  be  identified,  either  with  spark  or  arc  spectra. 

Corrections  to  Rowland's  Wave  Lengths. 

It  has  been  shown  that  the  wave  lengths  assigned 
by  Rowland  must  be  altered.  His  system  is  based 

1  Three  faint  lines  attributed  to  oxygen  are,  however,  now  known 
to  be  solar.  See  note  on  preceding  page. 

95 


THE  SUN 


on  measurements  by  several  observers  of  the  wave 
length  of  the  yellow  sodium  lines.  Measurements 
by  the  interferometer  in  the  hands  of  Michelson, 
Fabry,  Perot,  Buisson,  and  other  experimenters 
have  shown  that  Rowland's  assumed  wave-length  at 


D  should  be  reduced  by  about 


This  change, 


30,000' 

though  considerable  as  wave  lengths  go,  would  be 
of  little  consequence,  if  Rowland's  system  was  self- 
consistent.  But  it  is  further  shown  that  the  differ- 
ence from  the  true  scale  differs  for  different  parts  of 
the  spectrum  about  as  follows: 

TABLE    VI.     Corrections  to  wave  lengths  in  Rowland's  Preliminary 
Table  of  Solar-Spectrum  Wave  Lengths. 


Wave  lengths  

3000 

3200 

3400 

3600 

3700 

3900 

4100 

4300 

4500 

4700 

Corrections  

-.106 

-.124 

-.148 

-.155 

-.140 

-.144 

-.152 

-.161 

-.172 

-.179 

Wave  lengths  

4900 

5100 

5300 

5400 

5600 

5800 

6000 
-.213 

6200 

6400 
-.209 

6500 
-.210 

Corrections  

-.176 

-.170 

-.172 

-.212 

-.218 

-.209 

-.212 

These  discrepancies  are  to  be  ascribed  largely  to 
certain  deficiencies  of  the  grating  as  a  means  of 
measuring  wave  lengths,  and  not  to  avoidable  inac- 
curacy of  Rowland's  work,  although  he  neglected 
certain  small  corrections  not  strictly  negligible.  An 
effort  is  now  (1910)  being  made,  with  international 
cooperation,  to  establish  a  consistent  and  highly 
accurate  system  of  wave  lengths.  The  results, 
while  not  yet  officially  announced,  can  hardly  differ 

96 


THE   PHOTOSPHERE 

appreciably  from  those  indicated  in  the  above  table 
of  corrections  to  Rowland's  wave  lengths. 

An  accurate  table  of  solar  wave  lengths  and  of  the 
wave  lengths  of  the  lines  of  all  the  chemical  elements 
constitutes  the  fundamental  groundwork  of  all  mod- 
ern spectroscopic  investigation.  What  the  great 
star  catalogues  are  to  astronomy,  the  wave  length 
tables  are  to  astrophysics.  On  them  are  based  in- 
vestigations of  motion  and  pressure  in  the  sun  and 
stars,  of  the  elements  present,  the  magnetic  fields 
which  exist,  the  possibility  of  anomalous  dispersion 
phenomena,  and  other  solar  and  stellar  conditions. 

LEVELS 

In  the  general  spectrum  of  the  solar  photosphere 
we  have  an  index  of  conditions  which  exist  in  a  layer 
practically  at  the  surface  of  the  sun,  for,  as  shown  by 
terrestrial  experiments,  it  takes  only  a  little  of  an 
absorbing  gas  to  produce  a  dark  line  in  the  spectrum. 
But  it  is  thought  that  a  difference  of  average  level 
exists  in  the  positions  of  the  layers  which  produce 
lines  of  different  elements,  and  even  different  lines  of 
the  same  element.  The  layer  of  the  sun  which  gives 
rise  to  the  dark  Fraunhofer  lines,  though  thin  rela- 
tively to  the  solar  radius,  may  yet  be  thought  of  as 
made  up  of  several  layers  of  differing  level.  Calcium 
lines  are  thought  to  represent  a  higher  level  than 
iron  lines,  and  hydrogen  lines  one  still  higher.  Yet 
further,  as  the  longer  wave  lengths  are  often  more 
readily  emitted  by  an  element  than  the  shorter  ones, 

97 


THE  SUN 

that  is,  are  emitted  at  lower  temperatures,  it  may  be 
that  a  red  line  of  an  element  on  the  whole  represents 
a  higher  level  than  a  violet  line  of  the  same  element. 
The  continuous  background  of  the  solar  spectrum 
represents  a  lower  average  level  than  any  of  the  spec- 
trum lines,  as,  of  course,  follows  from  Kirchhoff  and 
Bunsen's  principle.  However,  the  continuous  back- 
ground offers  less  opportunities  of  investigation 
than  the  lines,  so  that  less  can  be  learned  of  the 
levels  it  represents  than  of  the  so-called  "  reversing 
layer"  where  the  lines  are  formed.  The  lines  them- 
selves are  not  to  be  regarded  as  dark  except  by  con- 
trast. If  seen  against  a  black  ground  they  would  be 
dazzlingiy  bright,  but,  as  they  are  formed  in  the 
outer  and  cooler  layers  of  the  sun,  they  are  less 
bright  than  the  spectrum  background  against  which 
they  are  seen.  The  light  of  the  deeper  solar  layers 
cannot  get  out,  if  it  is  of  a  wave  length  where  great 
absorption  occurs,  as  is  the  case  in  the  Fraunhofer 

lines. 

PRESSURES 

The  effect  of  pressure  is  two-fold.  It  broadens 
lines  and  shifts  them  in  wave  length.  Generally  the 
effect  is  the  same  whether  a  gas  is  compressed  by  a 
like  or  a  foreign  mass  of  gas.  Pressure  shifts  can  be 
distinguished  from  velocity  shifts,  because,  while  the 
former  increase  on  the  whole  with  increasing  wave 
length,  they  affect  different  lines  of  the  same  ele- 
ment and  of  different  elements  with  shifts  of  quite 
arbitrarily  differing  amounts,  and  sotae  lines,  indeed, 

98 


THE  PHOTOSPHERE 


are  practically  unaffected;  velocity  causes  shifts 
which  differ,  it  is  true,  in  different  parts  of  the 
spectrum,  but  which  are  directly  proportional  to  the 
wave  lengths.  Several  investigations  have  been 
made  to  determine  the  pressures  prevailing  in  the 
reversing  layer.  Jewell,  in  1896,  by  examination  of 
grating  spectra,  found  that  for  most  solar  lines  the 
wave  lengths  are  greater  by  a  few  thousands  of  an 
Angstrom  than  the  corresponding  lines  in  the  arc 
spectrum  at  atmospheric  pressure.  He  found,  to  be 
sure,  many  anomalies  which  tended  to  throw  doubt 
on  the  explanation  of  these  shifts  as  due  to  pressure, 
but  the  following  estimates  of  the  pressure  in  the 
reversing  layer  are  given  by  Jewell,  Mohler,  and 
Humphreys  i1 


ELEMENT. 

Alumi- 
num. 

Cobalt. 

Silicon. 

Cal- 
cium. 

Chro- 
mium. 

Man- 
ganese. 

Iron, 
Nickel, 
Copper, 
each. 

Pressure  .... 

2atm. 

4 

4 

GorS1 

5 

6 

7 

1  Depending  on  what  group  of  lines  is  observed.  The  H  and  K 
lines,  however,  are  not  included,  nor  is  4227. 

In  1909  Fabry  and  Buisson  examined  numerous 
iron  lines,  mostly  between  wave  lengths  4,000  and 
4,500  Angstroms,  by  interference  methods,  and  dis- 
covered small  shifts  in  the  same  sense  as  found  by 
Jewell.  They  also  investigated  the  behavior  of  the 
anomalous  cases  and  explained  them  as  due  to  un- 
symmetrical  broadening  under  pressure.  They  con- 

1  Astrophysical  Journal,  vol.  iii,  p.  139,  1896. 
99 


THE  SUN 

eluded  that  the  solar  reversing  layer  for  iron  lines 
lies  under  a  pressure  of  5.5  atmospheres.  Evershed, 
however,  criticises  their  interpretation  of  the  behav- 
ior of  the  anomalous  lines,  and  thinks  the  evidence 
tends  to  show  that  the  pressure  is  less  than  one  atmos- 
phere. 


CONVECTION  CURRENTS 

Some  recent  measurements  of  Adams  indicate 
velocities  of  ascent  of  from  0.1  to  0.3  kilometers  per 
second  in  the  solar  layer  where  the  metallic  absorp- 
tion lines  are  formed.  This  seems  at  first  sight  hard 
to  accept,  because  what  goes  up  must  surely  come 
down  again,  so  that  we  might  suppose  there  would  be 
as  much  of  a  Doppler  effect  of  descent  as  of  ascent. 
But  in  this  connection  we  must  consider  the  temper- 
atures of  the  ascending  and  descending  currents. 
Adams  refers  to  unpublished  experiments  of  Fox 
which  indicate  the  brighter  areas  or  "  granulations " 
of  the  sun's  surface  as  yielding  a  spectrum  strong 
in  " enhanced"  or  high  temperature  lines,  and  the 
darker  spaces  or  " pores"  between  as  regions  of  "arc" 
or  low  temperature  lines.  Adams  finds  the  "  en- 
hanced" lines  indicate  maximum  velocities  of  ascent. 
He  argues  that  the  spectrum  would  be  predominat- 
ingly influenced  by  the  hotter  and  brighter  parts,  and 
as  these  are  shown  to  be  ascending  the  whole  spec- 
trum would  hence  be  indicative  of  ascent.  Evershed 
had  advanced  a  similar  argument  in  1902  to  account 
for  peculiarities  of  the  "flash  spectrum." 

It  is  to  be  supposed  that  vertical  circulation  may 

100 


THE   PHOTOSPHERE 

be  active  in  the  sun  because  the  interior  is,  of  course, 
hotter  than  the  exterior;  the  latter  is  continually 
being  cooled  by  radiation,  and,  being  thereby  made 
denser,  would  tend  to  fall.  Velocities  of  0.1  to  0.3 
kilometers  per  second  are,  to  be  sure,  greater  than 
those  of  any  winds  we  know  of  on  the  earth.  On  the 
earth,  moreover,  the  vertical  circulation  and  the 
winds  are  to  a  large  extent  due  to  the  variable  tem- 
perature conditions  depending  on  the  changes  from 
day  to  night  and  from  summer  to  winter.  As  the  sun 
has  neither  night  nor  day,  summer  nor  winter,  it  is 
to  be  regarded  rather  as  having  approximately 
reached  a  steady  state  of  affairs;  but  still,  in  con- 
sideration of  the  sun's  enormous  temperature,  Mr. 
Adams'  results  give  no  cause  for  surprise. 

St.  John  has  still  more  recently  published  a  beauti- 
fully accurate  study  of  the  displacements  of  the  cal- 
cium lines  H  and  K,  and  of  the  calcium  circulation  to 
be  inferred  thereby  in  the  sun.  He  distinguishes 
three  parts  of  each  of  these  broad  lines,  which  he  in- 
dicates by  the  subscripts  1,2,3.  K3  is  the  narrow  dark 
line  in  the  center,  K2  the  bright  lines  on  either  edge  of 
K3,  and  Kt  the  dark,  broad,  diffuse  edges  on  the  out- 
sides  of  the  K2  regions.  Similarly  for  H,  St.  John 
concludes:1  "The  calcium  vapor  producing  the  ab- 
sorption band  K3  in  the  solar  spectrum  has  a  descend- 
ing motion  over  the  general  surface  of  the  sun  of  1.14 
kilometers  per  second  in  the  mean.  .  .  .  The  cal- 
cium vapor  to  which  the  bright  emission  line  K2  is 

1  Contributions  of  the  Mount  Wilson  Solar  Observatory,  No.  48. 

101 


THE  SUN 

due  has  an  ascending  motion  over  the  general  surface 
of  the  sun  of  1.97  kilometers  per  second  in  the  mean. 
.  .  .  The  wave  lengths  of  K2  (mean  of  both  parts  of 
K2)  and  K3  reduced  to  the  limb  are  3933.667  and 
3933.665  respectively.  The  corresponding  wave 
length  in  the  arc  at  atmospheric  pressure  is  3933.667. 
The  mean  pressure  in  the  intermediate  emitting  layer 
is,  therefore,  approximately  one  atmosphere.  .  .  . 
The  shorter  wave  length  of  the  K3  line  may  be  inter- 
preted as  indicating  a  somewhat  lower  pressure  in  the 
upper  absorbing  layer,  though  the  smallness  of  the 
quantities  involved  does  not  permit  a  positive  con- 
clusion. ...  In  the  case  of  the  intermediate  and 
highest  levels  of  calcium  vapor  [there  is  indicated  an] 
absence  of  currents  of  appreciable  velocity  parallel  to 
the  solar  surface.  .  .  .  The  widths  of  the  H3  and  K3 
lines  at  the  center  [of  the  disk],  compared  with  the 
corresponding  widths  in  the  arc,  point  to  an  extremely 
small  quantity  of  the  calcium  vapor  in  the  upper 
levels  of  the  solar  atmosphere.  .  .  .  The  average 
appreciable  height  of  the  atmospheric  calcium  shown 
by  a  radial  slit  is  about  5,000  kilometers  above  the 
photosphere.  The  thickness  of  the  upper  absorbing 
layer  is  approximately  1,500  kilometers.  Allowing 
700  kilometers  for  the  reversing  layer,  the  emitting 
layer  would  have  a  thickness  of  approximately  3,000 
kilometers.  The  elevation  at  which  the  K  line  'is 
appreciable  is  about  500  to  600  kilometers  above  the 
level  at  which  the  H  line  ceases  to  show.  .  .  .  The  shift 
between  limb  and  center  is  0.015  Angstroms  for  the 

102 


THE  PHOTOSPHERE 

H3  line,  and  in  agreement  with  that  obtained  for  the 
K3  line." 

An  interesting  result  on  the  rotation  of  the  sun  as 
measured  by  the  line  K3  will  be  given  below. 

It  is  by  no  means  to  be  supposed  that  the  fact  of 
the  enormous  transfer  of  heat  from  within  the  gase- 
ous body  of  the  sun  to  the  exterior,  to  supply  that 
which  is  lost  by  radiation  to  space,  requires  us  to 
imagine  a  strong  'Vertical  circulation  to  carry  it  on. 
At  low  temperatures,  as  for  instance,  between  a  body 
at  boiling  temperature  and  one  at  freezing,  convec- 
tion is  rather  more  important  than  radiation  as  a 
means  of  transferring  heat ;  but  this  is  probably  not 
the  case  at  the  temperatures  prevailing  within  the 
sun.  For  radiation  increases  with  the  fourth  power 
of  the  temperature,  and  convection  by  no  means  at 
such  a  tremendous  rate  of  increase.  Hence,  as  the 
material  of  the  sun  is  probably  transparent,  we  must 
suppose  that  the  heat  from  within  the  sun  becomes 
available  at  the  surface  to  supply  the  losses  of  energy 
by  radiation  to  space  chiefly  by  a  process  of  internal 
radiation,  gradual  absorption  in  a  long  path  outward, 
and  reradiation  nearly  counterbalancing  the  absorp- 
tion. This  process  is  repeated  as  many  times  as  nec- 
essary, and  except  for  the  very  short  time  occupied  by 
absorption  and  reradiation,  is  performed  at  velocities 
of  nearly  186,000  miles  a  second,  and  produces  quick 
communication  of  energy  from  within  outward.1 

JSee  Schwartzchild,  "Ueber  das  Gleichgowicht  der  Sonnenatmos- 
phare,"  Gottingen  Nachr.,  Mathphys.  KL,  1906,  pp.  1-13.  Prof.  T.  J.  J. 
See  also  takes  this  view  of  the  function  of  internal  solar  radiation. 
9  103 


THE  SUN 

If  we  admit  that  Adams  has  shown  an  effective 
velocity  of  ascent  averaging  0.12  kilometers  per 
second  and  the  shifting,  thereby,  of  average  solar 
lines  of  wave  length  4200  by  0.0015  Angstroms 
toward  the  violet,  then  a  correction  must  be  applied 
to  Fabry  and  Buisson's  results  tending  to  increase  by 
one  atmosphere  the  supposed  pressures  in  the  re- 
versing layer.  Adams  has  investigated  by  a  purely 
differential  method  the  shifting  of  lines  between  the 
center  and  limbs  of  the  sun,  and  finds  that,  after 
correcting  his  results  for  this  supposed  velocity  of 
ascent,  there  remain  in  the  spectra  of  the  limbs  well- 
substantiated  displacements  towards  the  red,  which 
are  best  explained  by  ascribing  them  to  effects  of 
pressure.1  Hydrogen,  sodium,  calcium,  and  mag- 
nesium lines  show  almost  no  displacement.  Lines  of 
titanium,  vanadium,  and  scandium  show  moderate 
displacement,  and  those  of  iron  and  nickel  consider- 
able shifts,  averaging  .007  Angstroms.  Lines  of  the 
elements  of  high  atomic  weight  show  very  small  dis- 
placements, as  do  also  lines  strengthened  at  the 
limb.  Enhanced  lines,  as  a  class,  show  maximum 
displacements,  which  apparently  grow  with  the  de- 
gree of  enhancement  of  the  several  lines.  These,  at 
first  sight  highly  discrepant,  observations  harmonize 
beautifully  under  Adams'  clever  discussion,  which  we 
shall  reserve  till  we  come  to  the  chapter  on  solar 
theory.  Adams  confirms  Fabry  and  Buisson's  ob- 
servation that  the  violet  edges  of  lines  do  not  shift. 

1  Contributions  of  the  Mount  Wilson  Solar  Observatory,  No.  43. 

104 


THE  PHOTOSPHERE 


LIMB  SPECTRA 

The  spectrum  of  the  sun's  limb  is,  as  would  be  ex- 
pected from  the  general  darkening  of  the  sun  towards 
the  limb,  weaker  than  that  at  the  center.  In 
violet  light  eight  or  ten  times  as  long  photographic 
exposure  is  required  for  the  limb  as  for  the  center. 
This  ratio  is  reduced  to  four  or  five  for  red  light. 
But,  besides  this  general  effect,  the  Fraunhofer  lines 
are  much  altered,  especially  in  the  violet.  The 
stronger  lines  almost  completely  lose  their  side 
shadings  or  "wings"  in  the  limb  spectrum,  while  in 
sun-spot  spectra,  as  we  shall  see  in  Chapter  V,  the 
wings  have  increased  prominence.  As  against  this 
marked  difference  from  spot  spectra,  the  limb  spec- 
trum is  like  that  of  spots  in  having  similar  changes 
of  relative  intensity  of  lines,  so  that  lines  strengthened 
in  spots  are  strengthened,  though  in  less  degree,  at 
the  limb,  and  vice  versa.  As  in  the  spots,  the  so- 
called  spark  or  "enhanced"  lines  are  often  weakened 
at  the  limb.  The  H>  line  of  hydrogen,  on  the  con- 
trary, is  widened  and  perhaps  strengthened  at  the 
limb,  although  narrowed  and  weakened  in  spots. 

VARIATION   OF  THE   SUN'S  BRIGHTNESS 

The  variation  of  the  brightness  of  the  sun  from 
the  center  to  limb  is  much  more  readily  determined 
by  the  bolometer  than  by  the  photographic  plate. 
Fig.  25  shows  the  distribution  of  brightness  along  a 

105 


THE  SUN 


diameter  of  the  sun's  disk  for  rays  of  different  wave 
lengths.  The  reader  will  notice  how  great  the  con- 
trast in  brightness  between  center  and  edge  is  for 
the  shorter  wave  lengths.  This  fact  is  also  shown 


9792  82.5  75    65    55 


SOLAR  RADIUS 


\ 


\ 


LL 


FIG.  25. — BRIGHTNESS  ON  SOLAR  DISK 

by  the  following  table,  which  gives  the  brightness 
at  different  percentages  of  a  solar  radius  from  the 
center  of  the  solar  disk,  and  with  which  the  data  of 

Figs.  25  and  26  agree. 

106 


THE  PHOTOSPHERE 


TABLE  VII. — Distribution  of  radiation  over  the  sun's  disk. 


Fraction 

xvRQlllS 

0.00 

0.40 

0.55 

0.65 

0.75 

0.825 

0.875 

0.92 

0.95 

Wave  length 

I    I 

0/M.323 

144 

129 

120 

112 

99 

86 

76 

64 

49 

0.386 

338 

312 

289 

267 

240 

214 

188 

163 

141 

0.433 

456 

423 

395 

368 

333 

296 

266 

233 

205 

0.456 

515 

486 

455 

428 

390 

351 

317 

277 

242 

0.481 

511 

483 

456 

430 

394 

358 

324 

390 

255 

0.501 

489 

463 

437 

414 

380 

347 

323 

286 

254 

0.534 

463 

440 

417 

396 

366 

337 

312 

281 

254 

0.604 

399 

382 

365 

348 

326 

304 

284 

259 

237 

0.670 

333 

320 

308 

295 

281 

262 

247 

227 

210 

0.699 

307 

295 

284 

273 

258 

243 

229 

212 

195 

0.866 

174 

169 

163 

159 

152 

145 

138 

130 

122 

1.031 

111 

108 

105.5 

103 

99 

94.5 

90.5 

86 

81 

1.225 

77.6 

75.7 

73.8 

72.2 

69.8 

67.1 

64.7 

61.6 

58.7 

1.655 

39.5 

38.9 

38.2 

37.6 

36.7 

35.7 

34.7 

33.6 

32.3 

2.09£ 

14.0 

13.8 

13.6 

13.4 

13.1 

12.8 

12.5 

12.2 

11.7 

Wave  length 

of  Max. 

O/t.458 

0/*.467 

O/Li.471 

O/u.474 

0/*.478 

O/x.483 

O/ut.489 

O/x.496 

Of*.  505 

Following  the  lines  of  the  table  from  left  to  right, 
the  reader  may  note  .the  decrease  of  brightness  from 
the  center  of  the  sun  to  95  per  cent  of  the  radius  out- 
ward1. The  results  are  arranged  vertically  in  order 
of  wave  length,  and  the  numbers  have  been  so  ad- 
justed that,  by  taking  any  single  vertical  column,  as 
for  instance,  that  for  75  per  cent,  out  on  the  radius, 
the  reader  may  find  for  a  single  zone  of  the  sun  the 
distribution  of  brightness  on  a  uniform  scale  of  wave 
lengths  for  the  normal  spectrum  outside  the  earth's 
atmosphere.  The  data  as  regards  distribution  along 
the  radius  for  wave  length  0.323/*  are  from  results 

1  There  is  a  tendency  of  all  the  data  plotted  in  Fig.  25  to  show  a 
less  rapid  fall  of  brightness  from  95  to  97  per  cent  out  on  the  radius, 
than  would  be  expected.  This  may  be  due  to  error, 

107 


THE  SUN 

of  Schwartzchild  and  Villager,  who  obtained  them  by 
photographing  the  solar  image  formed  by  a  silvered 
lens.  The  remainder  of  the  data  are  from  the  bolo- 
metric  results  of  Abbot  and  Fowle. 

In  the  preceding  table  the  maximum  number  in 
each  vertical  column  is  indicated  by  black-faced 
type.  But  the  wave  length  intervals  are  not  small 
enough  to  show  accurately  in  this  manner  the  amount 
of  shifting  of  the  wave  length  of  maximum  radiation 
for  light  coming  from  greater  and  greater  distances 
from  the  center  of  the  sun's  disk.  By  means  of 
plotting  the  values,  we  find  that  the  true  wave 
lengths  of  maximum  intensity  are  as  given  in  the 
lower  line  of  the  table.  This  shows  a  shifting  of  the 
maximum  of  radiation  from  0.458^  at  the  center  of 
the  sun's  disk  to  0.505/*  at  95  per  cent  out  on  the 
radius.  We  shall  see  that  a  similar  shifting  of  the 
wave  length  of  maximum  radiation  occurs  between 
the  photosphere  and  the  umbra  of  a  sun  spot.  The 
dotted  curve  of  the  accompanying  Fig.  26  shows  the 
distribution  of  radiation  in  the  spectrum  for  light 
of  the  whole  sun's  disk  as  it  would  be  if  viewed 
outside  the  earth's  atmosphere.  Similar  curves 
are  given  also  in  Fig.  26  for  the  center  of  the 
sun's  disk  and  for  points  55,  82.5  and  95  per  cent 
of  the  radius  towards  the  limb.  No  account  is 
made  in  the  figures  of  the  Fraunhofer  lines  sepa- 
rately, although  collectively  they  doubtless  affect 
the  forms  of  the  curves,  especially  for  the  shorter 
wave  lengths. 

108 


THE  PHOTOSPHERE 


SOLAR  TEMPERATURES 

First  Method. 

These  five  energy  curves  of  Fig.  26  are  of  inter- 
est as  they  indicate  the  probable  temperatures  in 


CS     06     07    Q8    09     1.0     II      1.2 


FIG.  26. — ENERGY  SPECTRA  ON  SOLAR  DISK. 

the   photosphere.     Fr.om  Wien's  displacement  law 
( xmax.  T  =  2930)  given  in  Chapter  II,  we  may  find, 


by  substituting  the  values  indicated  for 

109 


the 


THE  SUN 


values  of  the  absolute  temperatures  for  which  a  per- 
fect radiator  would  give  the  same  wave  lengths  of 
maximum  radiation.  The  values  are  given  in  Table 
VIII. 

Furthermore,  as  the  five  curves  of  Fig.  26  are 
plotted  with  ordinates  proportional  to  intensities, 
and  abscissae  proportional  to  wave  length,  their  in- 
cluded areas  are  proportional  to  the  intensities  of 
the  emission  of  all  wave  lengths  combined,  as  emitted 
from  the  selected  regions  of  the  sun's  disk.  If  the 
total  emission  is  comparable  to  that  of  a  perfect 
radiator,  then,  by  Stefan's  law,  it  is  proportional  to 
the  fourth  power  of  the  temperature  of  the  emitting 
body.  Hence,  the  fourth  roots  of  the  areas  included 
by  the  five  given  curves  should  be  in  inverse  ratio  of 
the  wave  lengths  of  maximum  emission.  The  fol- 
lowing table  shows  in  its  fourth  and  sixth  lines  how 
the  matter  comes  out: 

TABLE  VIII. — Energy  spectrum  relations  over  the  sun's  disk. 


POSITION. 

-*. 

Whole 
Disk. 

Center. 

55% 

82.5% 

95% 

Wave  lengthof  max- 
imum 

0  M68 

0  M58 

O.A*471 

O.)u483 

0./*505 

2930 

Amax. 

6260° 

6400° 

6220° 

6070° 

5800° 

Ratios  by  maximum 

1.079 

1.104 

1.073 

1.047 

1.000 

Ratios  of  Areas  .... 

1.407 

1.620 

1.476 

1.249 

1.000 

Ratios     by    fourth 
roots  of  areas  .... 

1.090 

1.128." 

1.102 

1.057 

1.000 

'On  the  absolute  scale  of  Centigrade  degrees  water  freezes  at 
273°  and  boils  at  373°. 


110 


THE  PHOTOSPHERE 

The  greatest  disagreement  between  the  ratios 
through  maximum  and  through  the  fourth  roots  of 
the  areas  is  about  2^  per  cent. 

Second  Method. 

Another  method  of  estimating  the  probable  solar 
temperature  is  by  attempting  to  match,  as  well  as 
possible,  the  distribution  of  energy  in  the  whole 
range  of  solar  spectrum  with  the  distribution  compu- 
ted by  the  Wien-Planck  formula  given  in  Chapter 
II.  Referring  to  Fig.  17,  the  reader  will  find  in 
curves  B  and  A  the  distribution  according  to  Wien- 
Planck  in  the  spectrum  of  a  perfect  radiator  at 
6200°  and  7000°  C.  absolute,  and  also  in  curve  C  the 
energy  spectrum  for  the  general  solar  surface.  No 
account  is  made  in  the  computations  of  the  relative 
values  of  the  constant  Ci  and  the  solar  constant  of 
radiation.  The  6200°  curve  has  been  repeated  at 
B'  on  a  larger  scale  of  ordinates,  and  the  observed 
curve  also  repeated  at  C'  on  a  scale  nearly  matching 
that  of  B'.  The  observed  curve  falls  below  the  com- 
puted ones  in  the  ultra-violet,  but  this  discrepancy  is 
to  be  expected,  partly  because  the  ultra-violet  solar 
spectrum  is  crowded  with  lines  of  selective  absorp- 
tion. 

On  the  other  hand,  the  observed  curve  rises  above 
the  computed  ones  in  the  infra-red,  a  feature  to  which 
Professor  Bigelow  has  repeatedly  called  attention. 
It  has  just  been  said,  and  it  will  be  spoken  of  at 
greater  length  in  Chapter  VI,  that  the  rays  from. the 

111 


THE  SUN 

center  of  the  sun's  disk  seem  to  arise  from  a  source  at 
higher  temperature  than  those  emanating  from  the 
sun's  limb.  In  accordance  with  the  explanation  of 
this  phenomenon  which  will  be  advanced  in  Chapter 
VI,  it  would  be  expected,  also,  that  solar  rays  of  long 
wave  lengths  would  appear  to  come  from  sources 
of  higher  temperature  than  would  those  of  shorter 
wave  lengths.  If  so,  we  shall  thereby  understand 
why  the  infra-red  parts  of  curves  C  and  C'  (Fig.  17) 
rise  above  curves  A,  B  and  B',  respectively,  for 
curves  C  and  C'  do  not  represent  the  spectrum  of  a 
source  at  constant  temperature.  Their  infra-red 
parts  correspond  to  much  hotter  sources  than  do 
their  visible  and  ultra-violet  parts.1  It  is  evident, 
however,  that  the  7000°  curve,  except  in  the  ultra- 
violet, is  a  better  match  for  the  observations  than  the 
6200°  curve.  The  large  discrepancy  in  the  ultra- 
violet is  probably  due  in  part  to  the  general  tendency 
toward  lower  temperature  in  the  sun  for  short  wave 
length  rays,  but  far  more  to  the  throngs  of  Fraunhofer 
lines  in  that  region  of  spectrum,  which  though  not 
shown  separately,  very  greatly  affect  the  form  of  the 
curve. 

Third  Method. 

Pointing  seemingly  to  a  lower  solar  temperature 
than  those  we  have  considered  are  the  following 

1  The  accuracy  of  the  observed  curve  for  wave  lengths  beyond 
2/i  is  seriously  impaired  by  the  effect  of  terrestrial  water  vapor,  so 
that  no  conclusion  should  be  drawn  from  the  fact  of  the  falling  off 
.of  the  curve  in  this  region. 

112 


THE   PHOTOSPHERE 


facts.     As  recently  done  for  a  large  number  of  stars 
by  Wilsing  and  Scheiner,  we  may  compute  the  ap- 
parent temperature  of  the  sun  by  the  formula  : 
Ei  Xt 

log-E;=  -51°g-^ 

where  Ex  and  E2  are  the  intensities  of  energy  at  two 
wave  lengths  \  and  ^2,  C2  a  constant  for  which 
Wilsing  and  Scheiner  prefer  the  value  14200,  and  T 
the  absolute  Centigrade  temperature.  Taking  a 
number  of  values  of  the  intensity  within  a  given 
range  of  wave  lengths,  and  proceeding  according  to 
the  method  of  least  squares,  1  find: 


Wave  length 
range  

O.,i30- 
0./i50 

0./*35- 
0.^50 

O.)u50- 
O./tTO 

0.^80- 
l.juSO 

l./*00- 
l./u50 

l.julO- 

l./i50 

Temperature  .  .  . 

3932° 

5142° 

6900° 

4493° 

4006° 

3840°  ' 

The  falling  off  of  computed  temperatures  for  long 
wave  length  rays  is  due  to  the  fact  that  the  observed 
curve  of  Fig.  17  rises  less  rapidly  from  the  infra-red 
towards  shorter  wave  lengths  than  does  the  6200° 
curve,  and  far  less  rapidly  than  the  7000°  curve. 
But,  as  we  have  said,  and  in  accordance  with  a  line 
of  explanation  to  be  given  in  Chapter  VI,  we  may 
assume  that  as  the  wave  length  decreases  the  effec- 
tive source  of  radiation  approaches  the  exterior  of 
the  sun,  and,  therefore,  is  cooler.  Hence,  although 
the  effective  temperatures  of  emission  for  the  infra- 
red rays  are  probably  exceeding  7000°,  the  observed 
energy  curve  does  not  rise  towards  its  maximum 
from  the  infra-red  side  as  fast  as  does  the  7000° 

113 


THE  SUN 

curve,  because  each  successive  shorter  wave  length 
is  emitted  from  a  lower  average  temperature  than  its 
next  longer  neighbor,  and  is,  therefore,  less  intense 
than  it  would  otherwise  be.  In  the  ultra-violet, 
however,  we  may  consider  the  temperature  of  effec- 
tive emission  not  only  apparently,  but  really  far  be- 
low 7000°,  on  account  of  the  superficial  region  of  its 
origin. 

On  the  whole,  the  preceding  review  of  the  form  of 
the  solar  energy  curve  inclines  us  to  set  the  average 
temperature  of  the  photosphere  certainly  above 
6200°,  and  possibly  near  7000°. 

Fourth  method. 

It  will  be  shown  in  Chapter  VII  that  the  intensity 
of  solar  radiation  at  the  earth's  mean  distance  from 
the  sun  is  1.95  calories  per  square  centimeter  per 
minute.  From  Stefan's  law,  with  Kurlbaum's  con- 
stant (see  Chapter  II),  a  perfect  radiator  emits  radi- 
ant energy  from  each  square  centimeter  of  its  surface 
at  the  rate  of  76.8  X  10~12T*  calories  per  minute. 
The  radius  of  the  sun  being  696,000  kilometers,  and 
the  mean  radius  of  the  earth's  orbit  149,560,000  kilo- 
meters, we  would  have  the  following  equation  for  a 
perfect  radiator  of  uniform  absolute  temperature  T 
in  the  sun's  place : 

(696,000)2  X76.8  X  10~12T4  =  ( 149, 560, 000) 2  x  1.95 

From  this,  T  =  5860°  absolute  C.  As  this  value  falls 
below  those  obtained  previously,  we  may  suppose  the 

114 


CALCIUM  SPECTROHELIOGRAM,  H2.     (Ellerman.) 
1908,  April  30.         G.  M.  T.     12h  53m.          P.  S.  T.     4h  43m  P.M. 


THE   PHOTOSPHERE 

sun's  constant  of  emission  is  a  little  less  than  that  of  a 
perfect  radiator. 

An  observation  which  may  be  regarded  as  con- 
firmatory of  the  view  that  the  photosphere  falls  some- 
what short  of  perfect  radiating  power  is  stated  by 
Jewell  as  follows : l 

"  When  some  of  the  very  best  negatives  of  the  solar 
spectrum  are  carefully  examined,  it  is  found  that 
some  of  the  sharp-edged,  clean-cut,  and  unshaded 
lines  of  iron,  chromium,  manganese,  titanium,  etc., 
have  a  faint,  dark  shading  just  outside  the  edge  of  the 
line.  It  is  very  faint  and  difficult  to  observe  (only 
slightly  darker2  than  the  general  background  of  the 
solar  spectrum),  but  it  is  not  due  to  contrast,  as  it  is 
not  always  present.  It  is  a  difficult  observation  to 
make,  but  was  observed  sometime  before  the  explan- 
ation forced  itself  upon  me.  The  correct  explanation 
undoubtedly  is  that  this  faint,  dark  shading  (dark  in 
the  negative  [overbright  in  the  spectrum])  is  the  re- 
mains of  an  emission  line,  either  produced  at  the 
photosphere  or  lower  down  in  the  solar  atmosphere 
than  the  absorption  line. " 

This  interesting  observation,  which  has  been  con- 
firmed by  Evershed,  appears  to  indicate  that  the 
photospheric  radiation  in  general,  though  undoubt- 
edly coming  from  hotter,  because  deeper,  layers  than 
the  rays  within  the  influence  of  the  Fraunhofer  lines, 
yet  lacks  something  of  the  full  intensity  of  perfect  or 

1  Astrophysical  Journal,  vol.  Ill,  p.  99,  1896. 

2  Darker  in  the  negative,  brighter  in  the  spectrum. 

115 


THE  SUN 

" black-body"  radiation.  For  thus  it  might  occur 
that  deep-lying  (yet  not  the  deepest  lying)  metallic 
vapors  would  give  in  the  immediate  proximity  of 
their  lines  of  powerful  selective  emission  a  more  in- 
tense radiation  than  the  deeper  lying  and  hotter,  but 
intrinsically  less  strongly  emissive,  layers  of  the 
photosphere. 

Summary. 

In  all  of  these  ways  discussed  of  estimating  the 
solar  temperature,  we  have  to  go  on  the  hypothesis 
that  the  sun  is  a  perfect  radiator.  This  is,  of  course, 
very  unlikely,  but  if  the  sun's  radiating  power  is 
not  perfect,  then  its  temperature  must,  at  any  rate, 
exceed  that  (5860°  abs.)  calculated  by  the  fourth 
method  from  Stefan's  law  of  radiation.  It  is  scarcely 
less  probable  that  the  solar  temperature  exceeds  that 
(6260°  abs.)  calculated  by  the  first  method  through 
Wien's  displacement  law.  For  the  influences  tend- 
ing to  distort  the  form  of  the  solar  spectrum  energy 
curve  seem  to  be  of  a  kind  to  diminish  the  violet 
most,  and  thereby  to  shift  the  maximum  of  energy 
towards  the  red.  Hence,  we  conclude  that  there  is  a 
high  probability  that  the  average  temperature  of  the 
apparent  photosphere  exceeds  5860°  or  even  6260° 
of  the  absolute  Centigrade  scale,  and  may  be  as  high 
as  7000°  absolute  Centigrade. 

The  reader  may  be  disposed  to  question  whether 
a  difference  of  temperature  probably  exists  between 
the  center  and  edge  of  the  apparent  photospheric  disk, 

116 


HYDROGEN  (Ha)  SPECTROHELIOGRAM.     (Ellerman.) 
1908,  April  30.         G.  M.  T.  13  h  6  m.         P.  S.  T.  5  h  6  m   P.  M. 


; ,  f\  ;: ,  •.':::    ••;./.! 


THE  PHOTOSPHERE 

as  brought  out  in  Table  VIII,  but  this  matter  will 
be  further  discussed  in  Chapter  VI.  One  highly  in- 
teresting conclusion  seems  to  follow  from  the  fact  of 
the  enormously  high  temperature  of  the  photosphere, 
taken  in  connection  with  the  spectroscopic  proof  of 
moderate  pressures  in  the  reversing  layer.  This  con- 
clusion is  that  no  known  substances  can  exist  in  the 
photosphere  except  as  gases.1  It  has  generally  been 
held  that  the  photosphere  is  a  cloudy  layer.  If  so, 
the  materials  composing  the  clouds  are  not  known  to 
exist  on  the  earth. 

THE  SPECTROHELIOGRAPH 

When  we  examine  the  sun  visually  or  by  direct 
photography,  the  source  of  the  light  is  highly  com- 
plex. Many  chemical  elements,  existing  in  a  layer 
many  hundreds,  or  perhaps  thousands,  of  miles  deep 
take  part  in  sending  the  light.  After  tentative  trials 
in  the  early  days  of  the  spectroscope  and  of  photog- 
raphy, the  matter  of  obtaining  a  view  of  the  sun  in  the 
light  of  one  element,  and  substantially  at  one  level, 
was  taken  up  about  1890  by  Hale  and  by  Deslandres 
independently,  and  in  1891  Hale  first  employed  his 
spectroheliograph.  Deslandres  has  long  used  a  simi- 
lar principle,  but  with  intermittent  instead  of  con- 
tinuous displacement  of  the  view  over  the  solar  sur- 
face, in  his  "spectroscope  a  vitesse."  He  has  lately 
employed  the  spectroheliograph  itself  with  great 
success.  The  spectroheliograph,  as  explained  in 

1  See  also  Chapter  VI. 
117 


THE  SUN 

Chapter  II,  is  in  effect,  a  screen  which  cuts  off  all 
light  except  that  of  a  single  spectral  line,  and  enables 
the  observer  to  see  how  the  vapor  of  a  single  element 
lies  on  the  sun's  surface. 

We  shall  now  examine  some  beautiful  spectroheli- 
ographic  results  obtained  on  Mount  Wilson  by  Mr. 
Ellerman,  which  Mr.  Hale  has  kindly  allowed  me  to 
reproduce  here.  Plate  V  is  taken  with  the  spectro- 
heliograph  in  the  H2  line  of  calcium.1  Comparing  it 
with  the  direct  photograph  of  the  sun  taken  on  the 
same  day,  shown  in  Plate  III,  at  the  beginning  of 
this  chapter,  there  is  seen  a  greater  distinctness  and 
prominence  of  detail.  Hale  has  called  the  mottlings 
shown  by  the  spectroheliograph  "flocculi, "  and  dis- 
tinguishes between  bright  and  dark  flocculi.  A  pho- 
tograph through  the  Ha  (C)  line  of  hydrogen,  made 
within  a  few  minutes  of  Plate  V,  is  given  in  Plate  VI. 
The  hydrogen  flocculi  are  generally  of  more  well- 
defined  shapes  than  the  calcium  flocculi,  and  usually 
dark  where  these  are  bright.  Bright  hydrogen  floc- 
culi, however,  often  appear  in  sun  spot  and  active 
regions,  and  such  bright  flocculi  frequently  change  in 
form  with  eruptive  rapidity. 

In  a  broad  line,  like  the  H  or  K  lines  of  calcium,  the 
slit  of  the  spectroheliograph  may  be  set  in  several 
positions.  Hale  distinguishes  three  such,  which  he 
terms,  HI,  H2,  H3  or  Kb  K2,  and  K3.  In  an  eclipse 

1  The  faint  structure  of  parallel  lines  seen  on  all  spectroheliographic 
plates  is.  not  a  solar  feature,  but  is  caused  by  very  slight  irregu- 
larities of  the  motion  of  the  instrument. 

118 


m 


THE   PHOTOSPHERE 

photograph  of  the  chromosphere  with  radial  slit  (see 
Chapter  IV)  the  H  and  K  lines  have  frequently  an 
" arrow  head"  appearance.  That  is:  The  light  of 
the  center  of  H  or  K  is  found  at  a  high  level  above  the 
sun,  and  the  matter  which  produces  the  light  of  the 
edges,  or  wings,  does  not  extend  out  so  far.  H3  or 
K3  corresponds  to  the  center  of  H  or  K  (seen  as  a 
dark  line  in  the  solar  spectrum).  Thus,  when  we 
look  at  a  K3  spectroheliographic  plate,  there  is  a 
deep  layer  of  calcium  vapor  behind  the  regions  shown, 
and,  as  it  takes  but  a  small  portion  of  this  thickness 
to  cut  off  by  absorption  the  light  of  this  wave 
length,  our  view  is  of  the  highest  levels  where  cal- 
cium occurs.  The  K2  and  Kt  positions  on  the  sides 
and  extreme  wings  of  K,  respectively,  correspond  to 
moderate  and  low  level  calcium  distribution.  In 
the  spectrum  of  hydrogen  a  similar  difference  of 
effective  level  in  spectroheliograph  observations 
is  attained  by  employing  lines  of  different  wave 
lengths.  In  eclipse  observations  high  hydrogen 
prominences  are  red,  owing  to  the  predominance  in 
their  light  of  rays  of  the  Ha  (C)  line.  Hence,  pho- 
tographs taken  through  the  Ha  (C)  line  give  high 
level  phenomena,  and,  as  might  be  plausibly  inferred 
from  a  consideration  of  Wien's  displacement  law, 
the  hydrogen  lines  of  successively  shorter  wave- 
lengths would  be  most  copiously  emitted  at  hotter, 
and  hence  lower  levels.  We  then  regard  an  Ha  or 
K3  pho'tograph  as  a  high-level,  an  H/3  or  K2  as  a 
medium,  and  an  H7  or  K!  as  a  low-level  phenomenon 
10  119 


THE  SUN 

for  hydrogen  and  calcium  respectively.  However, 
these  gases  are  both  high-level  gases  on  the  sun,  and 
the  photographs  of  the  sun  through  their  lines  are 
above  the  levels  where  most  Fraunhofer  lines  are 
produced.  It  is  to  be  expected  that  when,  with  in- 
creasingly powerful  instrumental  appliances,  the 
spectroheliograph  can  be  employed  in  the  narrower, 
and,  therefore,  more  difficult,  lines  of  the  heavier 
and  less  easily  vaporized  elements,  the  conditions  at 
lower  levels  will  be  shown. 

The  following  illustrations  bring  out  the  differences 
due  to  level  in  a  striking  manner.  Unfortunately, 
it  was  not  possible  for  Mr.  Ellerman  to  furnish  me  a 
series  showing  all  the  different  kinds  of  spectrohelio- 
grams  above  mentioned  for  a  single  day,  and,  indeed, 
it  was  found  necessary  to  omit  altogether  an  example 
of  H3  in  the  calcium  series.  Plates  VII  and  VIII 
show  a  spotted  area  of  the  sun's  surface  as  it  appeared 
July  16,  1907,  in  Hx  and  H2  calcium  spectrohelio- 
graphic  exposures.  Plates  IX,  X,  and  XI  illustrate 
a  spotted  region  of  the  solar  surface  as  it  appeared 
September  10,  1909.  They  are  taken  in  H2  of  cal- 
cium, Hy  and  Ha  of  hydrogen,  respectively.  In 
this  latter  series  the  first  plate  gives  no  hint  of  the 
pronounced  vortical  structure  revealed  by  the  high 
level  hydrogen  in  the  last  plate.  One  is  struck  by 
the  similarity  of  these  curved  structural  forms  to  the 
lines-of-force  diagrams  given  by  the  familiar  experi- 
ment of  shaking  fine  iron  filings  on  a  glass  plate  held 
horizontally  over  a  couple  of  magnets.  In  Chapter 

120 


SI 

0 

O  A 


W 


O 


THE  PHOTOSPHERE 

V  we  shall  have  occasion  to  refer  again  to  Plate  XI 
when  we  come  to  deal  with  the  magnetic  character 
of  the  sun  spots. 

The  spectroheliograph  results  will  receive  further 
attention  in  Chapter  IV  in  connection  with  the 
study  of  solar  prominences.  These  objects  are  great 
flamelike  protuberances  which  extend  for  thousands, 
sometimes  hundreds  of  thousands  of  miles  above  the 
photosphere.  First  observed  at  eclipses,  the  fact 
that  they  shine  principally  by  the  bright  spectrum 
lines  of  calcium  and  hydrogen  made  it  possible  to  see 
them  at  the  sun's  limbs  at  all  times  with  the  spec- 
troscope, and  now  the  spectroheliograph  has  enabled 
us  to  recognize  them  frequently  as  dark  hydrogen 
flocculi  on  the  disk  itself.  A  view  of  the  sun  through 
the  Ha  (C)  line  is  best  adapted  for  this  purpose,  and, 
indeed,  it  may  well  be  said  to  reveal  the  sun  in  quite  a 
new  aspect.  Direct  photographs  and  spectrohelio- 
graphic  results  through  H^  (C)  and  H  and  K  all 
show  a  mottling  of  the  solar  surfaces,  but  in  Plate 
XI  the  mottling,  especially  in  the  neighborhood  of 
sun  spots,  shows  a  marked  tendency  toward  curved 
and  spiral  forms,  as  if  the  hydrogen  at  this  high 
solar  level  were  definitely  arranged  by  cyclonic  mo- 
tions. Still  there  are  not  usually  found  observable 
motions  along  these  curved  lines,  although  in  ex- 
ceptional cases  series  of  Ha  spectroheliographic 
plates  have  given  evidence  of  definite  and  very  rapid 
motion.  Thus  St.  John  observing  on  Mount  Wilson 
on  June  3,  1908,  photographed  a  hydrogen  flocculus, 


THE  SUN 

probably  a  prominence,  apparently  moving  105,000 
kilometers  (60,000  miles)  in  18  minutes  towards  a 
double  sun  spot.  When  near  the  spot  the  flocculus 
divided,  and  apparently  each  branch  was  sucked 
into  a  sun  spot.  The  apparent  motion  in  this  case 
was  almost  exactly  radial  to  the  sun  spot  pair.  A 
dark  flocculus  of  a  similar  type,  which  is  also  probably 
a  prominence,  is  seen  in  Plate  VI.1 

THE  SOLAR  ROTATION 

The  rotation  of  the  sun  has  been  measured  by  ob- 
serving the  march  of  sun  spots,  faculae,  and,  of  late, 
spectroheliographic  flocculi  across  the  disk.  The 
classical  researches  of  Carrington  and  of  Spoerer  on 
the  march  of  sun  spots  showed : 

(1).  That  the  sun  rotates  about  an  axis  inclined 
about  7°  to  the  plane  of  the  ecliptic,  and  so  that  the 
sun's  axis  points  midway  between  the  polar  star  and 
Vega  to  a  position  in  right  ascension  18h  44m  and 
declination  64°. 

(2)  At  the  solar  equator  the  rotation  occurs  in 
about  25  days. 

(3)  The  period  of  one  rotation  increases  on  either 
side  of  the  equator  about  equally,  and  is  about  27)^ 
days  at  45°  north  or  south  solar  latitude. 

(4)  Individual  sun  spots  drift  in  different  directions 
on  the  sun's  surface,  so  that  it  is  only  the  mean  re- 

1  An  interesting  conclusion  relating  to  the  part  played  by  eruptive 
prominences  in  the  life  history  of  sun  spots  is  quoted  in  Chapter  V 
from  spectroheliographic  observations  of  Fox. 

122 


THE  PHOTOSPHERE 

suit  of  the  motions  of  many  spots  which  can  give 
accurately  the  solar  rotation  period. 

(5)  The  daily  rate  of  solar  rotation,  and  the  fact  of 
different  rotation  periods  for  different  solar  latitudes, 
were  both  expressed  by  Carrington  in  the  following 
formula,  in  which  X  is  the  daily  rate  of  rotation,  I 
the  solar  latitude: 

X  =  865'  -  -  165'  sin*  I 

Faye  assuming  on  theoretical  grounds  that  the 
exponent  of  sin  I  should  be  2,  derived  from  Carring- 
ton's  observations  of  1853-1861  the  expression: 

X  =  862'  --  186'  sin2 1 

Spoerer,  from  observations  of  his  own  between 
1862  and  1868,  combined  with  those  of  Secchi  and 
others,  obtained: 

X  -  1011'  -  203'  sin  (41°  13'  +  I) 
Tisserand    from  observations    of    1874-1875   ob- 
tained : 

X  =  857.6'  --  157.3' sin2  Z. 

Wilsing  and  later  Stratonoff  have  determined  the 
solar  rotation  from  observations  of  faculae.  As 
these  objects  can  seldom  be  followed  much  more  than 
a  quarter  way  across  the  solar  disk,  and  as  their 
appearance  is  usually  altered  when  they  reappear 
on  the  other  limb,  the  results  have  less  weight  than 
those  obtained  by  sun-spot  observations.  Wilsing 
found  no  evidence  of  equatorial  acceleration,  but 
Stratonoff  found  from'  the  faculae  similar  results  to 
those  of  Carrington  and  Spoerer  on  sun  spots.  Very 

123 


THE  SUN 

recently  Chevalier  has  published  results  of  a  long  and 
excellent  series  of  determinations  of  the  solar  rota- 
tion by  measurements  of  faculae.  His  work  con- 
firms that  of  Stratonoff. 

In  1908  Hale  published  determinations  of  solar 
rotation  from  spectroheliographic  plates  of  the  hydro- 
gen and  calcium  flocculi,  taken  through  the  HS  and 
H2  lines  respectively.  His  results  with  H2  calcium 
flocculi  are  in  close  agreement  with  those  obtained 
by  Fox  in  1903-4  for  the  same  line.  Their  results 
agree,  also,  at  all  latitudes  with  the  rates  of  solar 
rotation  derived  by  various  observers  from  observa- 
tion of  sun  spots.  With  HS  hydrogen  flocculi,  the 
rate  of  equatorial  rotation  was  about  the  same,  but 
there  was  found  no  retardation  at  higher  latitudes,  a 
fact  of  high  interest  and  significance. 

According  to  Doppler's  principle  the  spectral  lines 
of  a  source  receding  must  be  displaced  towards  the 
red  with  reference  to  those  of  a  source  approaching 
the  observer.  By  forming  the  solar  image  with  a 
telescope,  and  reflecting  light  from  the  two  limbs  si- 
multaneously upon  the  slit  of  a  spectroscope,  two 
spectra  may  be  produced,  one  immediately  above  the 
other,  which  exhibit  at  a  glance  the  shifting  of  all 
solar  lines  owing  to  the  sun's  rotation.  See  Plate 
IV.  Atmospheric  lines  are  not  thus  shifted. 

In  this  way  the  rate  of  solar  rotation  has  been  de- 
termined with  great  accuracy  by  Duner,  Halm,  and 
lately  by  Adams.  Their  results  bring  out  clearly  the 
fact  discovered  by  Carrington  from  the  study  of  sun 


I   fl 


• 


THE   PHOTOSPHERE 

spots,  namely,  that  the  sun's  angular  rotation  is 
slower  at  high  latitudes  than  at  the  sun's  equator. 
Since  sun  spots  do  not  occur  near  the  sun's  pole,  this 
peculiarity  could  not  be  thoroughly  studied  by  Car- 
rington,  but  by  the  spectroscopic  method  the  solar 
rotation  period  has  been  determined  at  high  as  well 
as  low  latitudes.  There  might  easily  be  a  doubt 
whether  the  spectroscopic  results  on  solar  rotation 
ought  to  agree  with  those  obtained  from  observing 
the  sun  spots,  the  faculae,  and  the  flocculi  exposed 
by  the  spectroheliograph,  for  the  sun,  as  indicated 
by  several  lines  of  evidence,  is  at  so  high  a  tempera- 
ture as  to  be  probably  almost  wholly  gaseous,  and 
our  vision  may  penetrate  to  some  distance  below  its 
surface.  The  several  objects  which  are  sources  of  the 
light  phenomena  employed  for  the  different  methods 
of  studying  the  solar  rotation  may  lie  at  different 
levels,  and  may,  therefore,  move  at  different  rates. 
Accordingly  it  is  interesting  to  compare,  in  the  fol- 
lowing table,  the  rotation  periods  indicated  by  the 
several  visual  methods  and  by  the  spectroscopic  ob- 
servations of  lines  of  different  chemical  elements. 
The  table  is  compiled  from  those  given  by  Hale  l 
and  by  Adams.2 

According  to  the  results  referred  to  in  the  following 
table,  the  sidereal  rotation  of  the  average  solar  sur- 
face is  completed  in  about  24.6  days  at  the  equator, 
26.3  days  at  ±  30°  latitude,  31.2  days  at  ±  60°, 

1  Contributions  of  the  Mount  Wilson  Solar  Observatory,  No.  25. 
wf.,  No.  33. 

125 


THE  SUN 


TABLE  IX. — Daily  rotation  of  the  sun's  surface. 
Various  methods  of  observing. 


Object 
observed.  -» 

Sun  spots. 

Faculse. 

Ca.  Flocculi 
H2  line. 

H.  Flocculi 
HS  line. 

Many  spec- 
tral lines. 

Observer.  -» 

Mean  of 
Carrington, 
Spoerer, 
Maunder. 

Mean  of 

Stratonoff 
and 
Chevalier. 

Mean  of 
Hale  and 
Fox. 

Hale. 

Adams,  1908. 
(Doppler 
effect.) 

Latitude,    i 

0°  to  ±  5° 

14.40° 

14.56° 

14.54° 

14.3° 

14.59° 

±  5     ±10 

14.35 

14.52 

14.41 

14.4 

14.48 

±10     ±15 

14.25 

14.33 

14.30 

14.6 

14.33 

±15     ±20 

14.13 

14.21 

14.13 

14.5 

14.15 

±20     ±25 

13.98 

14.19 

13.99 

14.7 

13.95 

±25     ±30 

13.80 

14.04 

13.97 

14.7 

13.74 

±30     ±35 

13.60 

13.  601 

13.75 

14.9 

13.50 

1  Stratonoff  only. 

Adams'  spectroscopic  results,  including  high  latitudes. 


Chemical 
elements.  -» 

Many. 

La,  (CN)a, 
(CN)8. 

Fe,  Ti, 
TiFe. 

Mn,  Fe, 
Fe. 

Ca. 

H. 

Wave 

lengths.  ~» 

Many. 

4196.699 
4197.257 

4265.418 
4287  .566 

4257  .815 
4290.542 

4226  .91 

6563.054 
(HaorC). 

Latitude.  I 

4216  .  136 

4288.310 

4291  .630 

0.3° 

14.65° 

14.49° 

14.65° 

14.72° 

15.0° 

15.2° 

14.9 

14.28 

14.21 

14.31 

14.34 

14.9 

15.0 

29.7 

13.66 

13.49 

13.65 

13.74 

14.2 

14.6 

44.7 

12.81 

12.74 

12.85 

12.95 

13.6 

14.0 

60.0 

11.52 

11.35 

11.53 

11.62 

12.5 

13.7 

74.9 

10.84 

10.50 

10.93 

11.04 

13.1 

14.3 

and  35.3  days  at  =*=  80°.  The  agreement  between 
Adams'  and  Duner's  work,  done  in  different  years, 
is  so  exact  that  there  seems  little  reason  to  suspect 
a  secular  variation  of  the  retardation  towards  high 
latitudes.  Adams  finds  his  mean  results  and  those 
of  Duner  and  Halm  well  expressed  by  the  following 
formula : 

f  =  10°.62  +  3°.99cos2  <£. 
126 


THE  PHOTOSPHERE 

Where  f  is  the  angular  sidereal  rotation  per  day  and 
$  the  solar  latitude. 

The  highly  interesting  change  in  the  observed 
rotation  period  for  lines  of  different  chemical  ele- 
ments is  regarded  as  indicating  differences  of  effec- 
tive level  of  the  production  of  the  Fraunhofer  lines. 
The  results  in  this  direction  gain  added  value  be- 
cause they  agree  with  several  other  lines  of  evidence 
that  point  to  the  same  conclusions.  The  whole 
subject  will  be  discussed  in  Chapter  VI. 

Very  recently  St.  John  has  determined  the  rate  of 
daily  rotation  spectroscopically  in  the  K3  line  of 
calcium.1  He  says:  "  The  angular  velocity  of  the 
high-level  calcium  producing  the  absorption  line  K3 
is  nearly  constant  for  the  latitudes  of  observation, 
being  15°.5  and  15°.4  per  day  at  the  latitudes  6°.6  and 
38°.4,  respectively.  The  corresponding  values  de- 
duced from  Adams'  results  are  15°.  1  and  14°.3  for 
hydrogen,  and  14°.4  and  13°. 2  for  the  reversing  layer. 
The  high  velocity  of  the  calcium  vapor  producing  the 
K3  line  points  to  a  higher  elevation  of  this  layer  of 
calcium  vapor  than  of  the  hydrogen  effective  in  the 
production  of  the  Ha  line."  It  is  a  very  singular 
thing  that  calcium  occurs  at  such  very  high  levels  in 
the  sun.  We  shall  see  the  fact  confirmed  in  the  next 
chapter,  but  the  reason  for  it  is  one  of  those  many 
puzzles  which  whet  the  appetite  of  the  student  in 
solar  research. 

1  Contributions  of  the  Mount  Wilson  Solar  Observatory,  No.  48. 

127 


CHAPTER   IV 

ECLIPSES    AND    THE    OUTER   SOLAR   ENVELOPES 

The  Saros. — Eclipse  Expeditions. — The  Corona. — The  Chromo- 
sphere.— The  Eclipse  of  1868  and  Jansen's  and  Lockyer's 
Discovery. — Spectrum  of  the  Chromosphere  and  Prominences. — 
Prominences  and  the  Spectroheliograph. — Recent  Flash  Spec- 
trum Observations. — The  Heights  of  Different  Metals  in  the 
Chromosphere. — Mitchell's  Observations  of  1905. — Campbell's 
Observations. — Chromospheric  Spectra  in  Full  Daylight. 

WHEN  the  moon  passes  directly  between  the  earth 
and  the  sun  it  sometimes  completely  covers  the 
latter,  and  there  is  a  total  solar  eclipse.  At  such 
times  the  brilliant  glare  of  day  ceases  for  a  few  mo- 
ments to  illuminate  our  atmosphere,  and  in  the  semi- 
darkness  we  may  see  the  objects  which  closely  sur- 
round the  sun.  Total  solar  eclipses  occur  almost 
every  year,  but  as  the  moon  is  never  much  greater  in 
angular  diameter  than  the  sun,  the  area  of  the  earth's 
surface  on  which  the  eclipse  appears  total  at  a  given 
instant  is  rarely  greater  than  100  miles  in  average 
diameter.  The  rapid  motion  of  the  moon,  though 
partly  offset  by  the  rotation  of  the  earth,  hurries  the 
region  of  totality  along  faster  than  1,000  miles  an 
hour,  making  a  belt  seldom  wider  than  100  miles,  but 
sometimes  more  than  5,000  miles  long,  on  which  the 
eclipse  is  total  at  sometime  between  sunrise  and  sun- 

128 


ECLIPSES   AND   SOLAR   ENVELOPES 

set.  Over  enormous  areas  on  either  side  of  the  line 
of  totality  the  sun  is  partially  eclipsed,  and  appears 
for  some  hours  as  a  crescent  figure. 

THE  SAROS 

The  ancients  discovered  a  cycle  of  eclipses  called 
the  Saros,  which  indicates  approximately  the  times 
when  solar  eclipses  will  occur.  In  223  synodic 
months  there  are  almost  exactly  nineteen  "  eclipse 
years"  of  346.62  days,  the  interval  between  the  times 
when  the  sun  in  its  apparent  annual  path  crosses  the 
two  nodes  of  the  moon's  orbit.  Hence,  if  we  count 
forward  6,585  days,  or  18  years  11  days,  from  one 
total  eclipse,  we  are  apt  to  find  the  occurrence  of 
another  either  partial  or  total.  A  family  of  eclipses 
thus  occurs,  separated  by  intervals  of  about  eighteen 
years.  Such  a  family  generally  numbers  about  sixty- 
five  or  seventy  eclipses,  of  which  perhaps  eighteen 
will  be  total,  and  the  rest  annular  or  partial.  Many 
total  eclipses  are  visible  only  at  regions  unfavorable 
for  observation,  such  as  oceans,  the  polar  regions,  or 
very  cloudy  localities.  As  totality  at  a  given  place 
never  lasts  more  than  eight  minutes,  and  generally 
does  not  exceed  three,  there  have  been  hardly  more 
than  a  couple  of  hours  of  time  employed  in  total  solar 
eclipse  observing  in  the  last  half  century.  Yet  so  well 
have  the  moments  been  utilized  that  a  large  stock  of 
information  has  been  gathered. 

Not  infrequently  eclipse  expeditions  have  led  as- 
tronomers to  experiences  of  hardship,  disappointment 

129 


THE   SUN 

and  in  one  instance  to  death.  Father  Perry,  of  Stony- 
hurst,  who  led  the  English  eclipse  expedition  to  Cay- 
enne, in  1889,  was  taken  ill  before  the  awaited  day. 
He  insisted  on  observing,  supported  by  an  attendant, 
and  called  for  three  cheers  when  the  eclipse  had  been 
successfully  observed,  saying:  "I  can't  cheer,  but  I 
will  wave  my  helmet!"  A  few  days  later  he  died  at 
sea. 

A  total  eclipse  having  been  predicted  to  occur  at  a 
certain  place  favorable  for  observing,  astronomers 
journey  there  several  weeks  in  advance,  equipped 
with  photographic  telescopes,  spectroscopes,  auxil- 
iary apparatus,  and  supplies.  The  instruments  are 
set  up  and  carefully  adjusted  and  are  provided  with 
every  possible  contrivance  to  facilitate  and  shorten 
their  operation  at  the  critical  moment.  Rehearsals 
of  the  eclipse  begin  as  soon  as  possible.  Time  signals 
are  counted  off,  photographic  apparatus  is  manip- 
ulated, and  the  whole  program  is  gone  over  and  over 
again,  just  as  if  the  totality  were  on.  In  this  way 
the  observers  try  to  anticipate  all  possible  contin- 
gencies, and  acquire  skill  and  rapidity  in  performing 
their  parts.  Mr.  Langley  used  to  say  that  if  a  pin 
were  likely  to  be  dropped  during  the  eclipse  the  ob- 
server should  practice  dropping  one  and  filling  its 
place  at  rehearsal.  The  hour,  minute,  and  second  of 
the  eclipse  are  predicted  long  in  advance,  so  that  on 
the  appointed  day  all  is  prepared  for  action  at  a  well 
known  time.  At  first  contact  of  the  moon  a  notch 
begins  to  appear  in  the  sun's  disk,  and  this  grows 

130 


ECLIPSES  AND  SOLAR  ENVELOPES 

larger  and  larger  during  the  next  hour  and  a  half, 
until  only  a  narrow  crescent  remains.  This  hour 
and  a  half  has  always  seemed  to  the  writer  the  saving 
element;  for  during  its  slow  passage  the  unhurried 
march  of  events  tends  to  calm  the  nervous  agitation 
which  comes  on  with  the  first  contact,  when  one  feels 
that  his  opportunity  is  now  or  never.  As  the  cres- 
cent becomes  thin  the  sun's  light  becomes  noticeably 
weak  and  yellow,  for  only  the  limb  now  remains  vis- 
ible, and  its  light,  as  stated  already,  is  very  much 
weaker,  especially  in  the  violet  end  of  the  spectrum, 
than  the  light  of  the  center  of  the  disk.  Just  before 
totality  flickering  bands,  called  " shadow  bands," 
steal  rapidly  along  the  ground,  and  then,  as  the  last 
crescent  of  the  photosphere  suddenly  vanishes,  a  thin 
ring  of  rosy  light  encircles  the  moon,  and  beyond  this, 
for  perhaps  one  or  even  two  diameters  of  the  sunr 
blooms  forth  the  pearly  hued  corona. 

THE  CORONA 

There  is  a  cycle  of  changes  in  the  form  of  the 
corona  having  a  period  of  about  eleven  years,  sup- 
posed to  be  identical  with  that  of  sun-spot  frequency, 
which  will  be  noticed  in  Chapter  V.  As  the  corona 
can  be  observed  only  at  total  solar  eclipses,  the  march 
of  the  cycle  of  changes  is  as  yet  only  imperfectly 
known,  but  for  the  last  half  century  it  has  been 
observed  that  there  are  long  equatorial  coronal 
streamers  at  the  time  of  sun-spot  minimum,  while 
at  maximum  of  sun  spots  the  corona  extends  only 

131 


THE  SUN 

to  moderate  distances,  but  nearly  uniformly  in  all 
directions  from  the  sun.  The  accompanying  views, 
Plate  XII  from  a  drawing  of  Mr.  P.  R.  Calvert  pre- 
pared from  the  Yerkes  Observatory  photographs  of 
the  1900  eclipse,  Plate  XIII  from  a  drawing  of  Mrs. 
C.  G.  Abbot  prepared  from  U.  S.  Naval  Observa- 
tory photographs  of  the  1905  eclipse,  illustrate  the 
characteristic  forms  of  the  corona  at  sun-spot 
minima  and  maxima  respectively. 

Many  efforts  have  been  made,  but  thus  far  with- 
out success,  to  devise  a  method  of  observing  the 
corona  without  an  eclipse.  Success  is  unlikely,  for 
in  its  brightest  parts,  even  within  y1^  radius  of  the 
sun's  limb,  the  brightness  of  the  corona  is  only  about 
one-tenth  as  great  as  that  of  the  daylight  sky  at 
20°  from  the  sun,  if  viewed  from  sea  level.  Close 
to  the  sun  the  daylight  sky  is  many  fold  brighter 
still,  so  that  the  coronal  brightness  is  insignificant 
in  comparison  with  it.  By  ascending  a  very  high 
mountain,  it  is  true,  a  considerable  gain  might  be 
made,  for  the  corona  would  be  a  little  brighter  and 
the  sky  several  fold  less  bright,  but  the  brightness 
of  the  sky  would  still  be  far  too  great  to  permit  the 
corona  to  be  seen,  even  in  its  brightest  parts,  by  any 
contrivance  yet  devised. 

The  corona  fades  rapidly  with  increasing  distance 
from  the  sun.  According  to  Turner,  who  has  dis- 
cussed results  of  various  eclipses,  it  falls  off  approxi- 
mately as  the  sixth  power  of  the  distance  from  the 
sun's  center.  L.  Becker  has  discussed  photographic 

132 


ECLIPSES  AND  SOLAR  ENVELOPES 


observations  made  at  the  eclipse  of  1905,  and  gives 
the  following  formula  of  distribution  of  the  intensity 
of  the  blue  and  violet  coronal  radiation  at  different 
distances,  H,  from  the  sun's  limb;  I  is  the  intensity, 
C  is  a  constant,  and  H  is  expressed  in  thousandths 
of  a  solar  diameter. 

I  =  C  (H  +  140)-4. 

At  the  eclipse  of  January  3,  1908,  the  present  writer, 
assisted  by  A.  F.  Moore,  made  bolometric  observa- 
tions of  the  intensities  of  the  coronal  radiation  at 
several  distances  from  the  sun's  limb.  These  were 
made  both  with  and  without  a  screen  of  asphaltum 
varnish  on  glass.  This  screen  was  used  to  cut  off 
the  visible  spectrum  while  still  transmitting  the 
infra-red.  The  following  is  a  comparison  of  these 
results  with  those  computed  according  to  the  for- 
mulae of  Turner  and  of  Becker. 


H  = 

45. 

121. 

364. 

Total  radiation 

100 

29  9 

o 

Visible  radiation 

100 

29  8 

o 

Infra-red  radiation             .      .  . 

100 

30  1 

o 

Computed  via  Becker  
Computed  via  Turner  

100 
100 

25.2 
45.7 

1.8 
6.3 

The  agreement  between  the  bolometric  observations 
and  the  computation  by  Becker's  formula  is  pretty 
good,  so  that  for  a  sun-spot  maximum  corona  it 
seems  to  represent  the  distribution  for  all  kinds  of 
radiation,  at  least  in  the  inner  corona. 

The  light  shows  distinct  radial  polarization  in  the 

133 


THE  SUN 

outer  corona,  but  the  percentage  of  polarization  de- 
creases, and  at  length  vanishes  near  the  limb  of  the 
sun.  Polarization  of  the  coronal  light  is  generally 
interpreted  as  evidence  of  the  presence  of  reflected 
photospheric  rays  in  the  coronal  brightness,  just  as 
sky-light  and  its  polarization  is  produced  by  the  dif- 
fuse reflection  of  sunlight  in  the  air.  Some  writers 
have  inferred  from  the  absence  of  polarization  near 
the  sun's  limb  that  the  light  of  that  part  of  the  corona 
contains  almost  no  reflected  photospheric  rays.  But 
a  particle  near  the  sun's  limb  must  be  shone  upon 
from  every  direction  within  a  hemisphere,  so  that 
the  light  which  it  reflects,  being  partially  polarized 
in  every  plane,  would  show  polarization  in  none. 
Hence,  the  absence  of  reflected  photospheric  rays 
from  the  inner  coronal  brightness  cannot  rightly  be 
inferred  from  the  absence  of  polarization. 

The  spectrum  of  the  corona  is  more  nearly  con- 
tinuous than  that  of  the  photosphere.  A  few  bright 
lines  are  found,  but  these  are  not  conspicuous  at  most 
eclipses.  There  is  a  famous  bright  coronal  line  in 
the  green  at  wave  length  5303.  This  line  was  dis- 
covered by  Young,  in  1870,  and  it  has  been  seen  with 
more  or  less  distinctness  at  many  subsequent  eclipses. 
It  does  not  correspond  in  wave  length  to  a  line  of 
any  known  substance,  or  to  a  photospheric  line,  so 
that  it  is  ascribed  to  a  hypothetical  element  "coro- 
nium."  As  the  element  helium  was  found  in  the 
earth  after  its  spectrum  had  long  been  known  in  the 
sun  and  stars,  so  it  may  happen  with  "coronium." 

134 


PLATE  XIII 


SOLAR  CORONA.     1905,  AUGUST  30. 

From  Drawing  by  Mrs.  C.  G.  Abbot  from  Photographs  by  the  United  States 
Naval  Observatory  Eclipse  Expedition. 


ECLIPSES  AND  SOLAR  ENVELOPES 

Several  bright  coronal  lines  have  been  discovered 
in  the  ultra-violet  by  Deslandres,  Dyson,  Lewis,  and 
others.  In  the  outer  corona  the  Fraunhofer  lines 
of  the  photospheric  spectrum  have  been  seen,  and 
have  been  repeatedly  photographed  by  Campbell, 
Perrine  and  others.  Lewis  found  them  only  in  the 
ultra-violet  spectrum  in  the  eclipse  of  1908.  These 
dark  lines  fade  and  disappear  near  the  limb  of  the  sun. 
Their  presence  in  the  outer  corona  is  a  proof  of  the 
presence  in  the  outer  coronal  light  of  a  large  'pro- 
portion of  reflected  photospheric  rays,  but  Campbell 
infers  from  their  absence  near  the  sun's  limb  that 
the  inner  corona  shines  almost  wholly  by  light  of 
incandescence  of  the  material  there,  due  to  its  being 
heated  by  proximity  to  the  sun.  There  are,  however, 
several  causes  other  than  a  great  admixture  of  coronal 
light  of  incandescence  which  must  contribute  to 
diminish  the  distinctness  of  the  Fraunhofer  lines  of 
the  inner  corona  near  the  sun's  limb.  Among  these 
are  (1)  atmospheric  reflection  of  the  strong  bright 
line  spectrum  of  the  chromosphere;  (2)  over  ex- 
posure of  photographic  spectra  for  the  very  inner- 
most corona,  etc. 

It  was  inferred  by  Bigelow  and  by  Holden,  from 
studies  of  eclipse  photographs,  that  the  corona  par- 
ticipates in  the  rotation  of  the  sun.  This  view  is  con- 
firmed by  spectroscopic  observations  of  Deslandres, 
Campbell,  and  Belopolsky.  It  has  been  supposed  by 
many  that,  as  the  polar  streamers  of  the  corona  ap- 
pear much  like  terrestrial  auroras  seen  in  high  north- 
11  135 


THE  SUN 

ern  and  southern  latitudes,  the  corona  may  have,  like 
them,  an  electrical  origin.  They  would  regard  its 
light,  like  that  of  the  aurora,  as  largely  of  lumines- 
cence similar  to  that  of  a  glow  electrical  discharge, 
and  not  true  temperature  radiation.  The  writer's 
bolometric  observations  of  the  inner  corona  at  the 
eclipse  of  1908  seemed  to  be  incompatible  with  the 
view  that  it  shines  mainly  by  light  of  ordinary  incan- 
descence. For  by  the  aid  of  absorbing  screens  it  was 
shown  that  the  ratio  of  intensity  of  infra-red  to  total 
radiation  is  almost  the  same  for  the  inner  corona  as 
for  the  photosphere.  If  the  corona  shines  mainly  by 
incandescence,  and  its  high  temperature  is  produced 
by  the  absorption  of  sunlight  in  its  particles,  then  the 
fraction  of  its  radiation  occurring  in  the  infra-red 
spectrum  should  be  disproportionately  greater  than 
that  for  the  photosphere,  because  the  temperature  of 
the  corona  must  be  much  the  lower.  Lewis,  however, 
at  the  same  eclipse  found  the  ultra-violet  coronal 
rays  disproportionately  weaker  than  those  of  the 
photosphere,  and  inferred  therefrom  a  low  coronal 
temperature.  The  composition  of  the  inner  coronal 
light  cannot  yet  be  regarded  as  settled.  There 
is  undoubtedly  some  reflected  light,  some  light 
of  incandescence,  and  perhaps  some  of  lumines- 
cence. It  may  be  that  it  is  the  latter  which  is 
the  key  to  the  perplexing  observations  above  re- 
corded. The  nature  of  the  corona  will  be  further 
discussed  in  Chapter  VI. 


136 


ECLIPSES  AND  SOLAR  ENVELOPES 

THE  CHROMOSPHERE 

Close  to  the  limb  of  the  sun  there  is  seen  at  total 
solar  eclipses,  and  by  special  contrivances  also  in  full 
sunlight,  a  thin  ring  of  rosy  light  called  the  "  chro- 
mosphere, "  from  which  project  irregularly,  some- 
times as  much  as  50,000  or  even  100,000  miles,  rosy 
forms  called  "  prominences. "  The  spectrum  of  the 
chromosphere  consists  of  bright  lines  on  a  faint  con- 
tinuous background.  These  bright  lines  are  the 
counterparts  in  position,  and  generally,  also,  in  rela- 
tive intensity,  of  the  dark  Fraunhofer  lines  of  the 
photospheric  spectrum.  The  prominences  appear 
to  be  but  higher  extensions  of  the  chromosphere, 
yet  their  spectra  are  usually  simpler.  Prof.  C.  A. 
Young  made  a  prolonged  study  of  the  prominences 
and  of  their  spectra,  and  I  cannot  introduce  the  mat- 
ter better  than  to  quote  his  descriptions  (pages  197 
to  226  of  "The  Sun")  supplementing  his  story  by 
mention  of  most  recent  work  on  the  subject.  Young's 
explanations  of  some  of  the  phenomena  differ  some- 
what from  those  which  the  present  writer  would  pre- 
fer, as  Young  was  a  believer  in  the  cloudy  photo- 
sphere. 

The  Eclipse  of  1868.     Jansseri's  and  Lockyer's  Dis- 
covery. 

"Every  one  is  more  or  less  familiar  with  the  story 
of  this  eclipse.  Herschel,  Tennant,  Pogson,  Rayet, 
and  Janssen,  all  made  substantially  the  same  report. 

137 


THE  SUN 

They  found  the  spectrum  of  the  prominences  to  con- 
sist of  bright  lines,  and  conspicuous  among  them  were 
the  lines  of  hydrogen.  There  were  some  serious  dis- 
crepancies, indeed,  among  their  observations,  not 
only  as  to  the  number  of  the  bright  lines  seen,  which 
is  not  to  be  wondered  at,  but  as  to  their  position. 
Thus,  Rayet  (who  saw  more  lines  than  any  one  else) 
identified  the  red  line  observed  with  B  instead  of  C; 
and  all  the  observers  mistook  the  yellow  line  they  saw 
for  that  of  sodium. 

"Still,  their  observations,  taken  together,  com- 
pletely demonstrated  the  fact  that  the  prominences 
are  enormous  masses  of  highly  heated  gaseous  matter, 
and  that  hydrogen  is  a  main  constituent. 

"Janssen  went  further.  The  lines  he  saw  during 
the  eclipse  were  so  brilliant  that  he  felt  sure  he  could 
see  them  again  in  the  full  sunlight.  He  was  pre- 
vented by  clouds  from  trying  the  experiment  the 
same  afternoon,  after  the  close  of  the  eclipse;  but 
the  next  morning  the  sun  rose  unobscured,  and,  as 
soon  as  he  had  completed  the  necessary  adjustments, 
and  directed  his  instrument  to  the  portion  of  the 
sun's  limb  where  the  day  before  the  most  brilliant 
prominence  appeared,  the  same  lines  came  out  again, 
clear  and  bright;  and  now,  of  course,  there  was  no 
difficulty  in  determining  at  leisure,  and  with  almost 
absolute  accuracy,  their  position  in  the  spectrum. 
He  immediately  confirmed  his  first  conclusion,  that 
hydrogen  is  the  most  conspicuous  component  of  the 
prominences,  but  found  that  the  yellow  line  must 

138 


ECLIPSES   AND   SOLAR   ENVELOPES 

be  referred  to  some  other  element  than  sodium,1  being 
somewhat  more  refrangible  than  the  D  lines. 

"He  found  also  that,  by  slightly  moving  his  tele- 
scope and  causing  the  image  of  the  sun's  limb  to  take 
different  positions  with  reference  to  the  slit  of  his 
spectroscope,  he  could  even  trace  out  the  form 
and  measure  the  dimensions  of  the  prominences; 
and  he  remained  at  his  station  for  several  days, 
engaged  in  these  novel  and  exceedingly  interesting 
observations. 

"Of  course,  he  immediately  sent  home  a  report  of 
his  eclipse- work,  and  of  his  new  discovery,  but,  as  his 
station  at  Guntoor,  in  eastern  India,  was  farther  from 
mail  communication  with  Europe  than  those  upon 
the  western  coast  of  the  peninsula,  his  letter  did  not 
reach  France  until  some  week  or  two  after  the  ac- 
counts of  the  other  observers;  when  it  did  arrive,  it 
came  to  Paris,  in  company  with  a  communication 
from  Mr.  Lockyer,  announcing  the  same  discovery, 
made  independently,  and  even  more  creditably,  since 
with  Mr.  Lockyer  it  was  not  suggested  by  anything 
he  had  seen,  but  was  thought  out  from  fundamental 
principles. 

"Nearly  two  years  previously  the  idea  had  oc- 
curred to  him  (and,  indeed,  to  others  also,  though  he 
was  the  first  to  publish  it)  that,  if  the  protuberances 
are  gaseous,  so  as  to  give  a  spectrum  of  bright  lines, 
those  lines  ought  to  be  visible  in  a  spectroscope  of 

1  This  element  is  helium  and   was  discovered  on  the  earth  long 
afterwards. 

139 


THE  SUN 

sufficient  power,  even  in  broad  daylight.  The  prin- 
ciple is  simply  this : 

"  Under  ordinary  circumstances  the  protuberances 
are  invisible,  for  the  same  reason  as  the  stars  in  the 
daytime:  they  are  hidden  by  the  intense  light  re- 
flected from  the  particles  of  our  own  atmosphere  near 
the  sun's  place  in  the  sky,  and,  if  we  could  only  suf- 
ficiently weaken  this  aerial  illumination,  without  at 
the  same  time  weakening  their  light,  the  end  would 
be  gained.  And  the  spectroscope  accomplishes  pre- 
cisely this  very  thing.  Since  the  air-light  is  re- 
flected sunshine,  it  of  course  presents  the  same  spec- 
trum as  sunlight,  a  continuous  band  of  color  crossed 
by  dark  lines.  Now,  this  sort  of  spectrum  is  greatly 
weakened  by  every  increase  of  dispersive  power,  be- 
cause the  light  is  spread  out  into  a  longer  ribbon  and 
made  to  cover  a  more  extended  area.  On  the  other 
hand,  a  spectrum  of  bright  lines  undergoes  no  such 
weakening  by  an  increase  in  the  dispersive  power  of 
the  spectroscope.  The  bright  lines  are  only  more 
widely  separated — not  in  the  least  diffused  or  shorn 
of  their  brightness.  Moreover,  if  the  gas  is  one  which, 
like  hydrogen,  shows  dark  lines  in  the  ordinary  solar 
spectrum  (and  therefore  in  that  of  the  air-light), 
the  case  is  even  better:  not  only  is  the  continuous 
spectrum  of  the  air-light  weakened  by  the  high  dis- 
persion, but  it  has  dark  gaps  in  it  just  where  the 
bright  lines  of  the  prominence  spectrum  will  fall. 

"If,  then,  the  image  of  the  sun,  formed  by  a  tele- 
scope, be  examined  with  a  spectroscope,  one  might 

140 


ECLIPSES  AND   SOLAR  ENVELOPES 

hope  to  see  at  the  edge  of  the  disk  the  bright  lines 
belonging  to  the  spectrum  of  the  prominences,  in  case 
they  are  really  gaseous. 

"Mr.  Lockyer  and  Mr.  Huggins  both  tried  the  ex- 
periment as  early  as  1867,  but  without  success;  partly 
because  their  instruments  had  not  sufficient  power  to 
bring  out  the  lines  conspicuously,  but  more  because 
they  did  not  know  whereabouts  in  the  spectrum  to 
look  for  them,  and  were  not  even  sure  of  their  exis- 
tence. At  any  rate,  as  soon  as  the  discovery  was  an- 
nounced, Mr.  Huggins  immediately  saw  the  lines 
without  difficulty,  with  the  same  instrument  which 
had  failed  to  show  them  to  him  before.  It  is  a  fact, 
too  often  forgotten,  that  to  perceive  a  thing  known 
to  exist  does  not  require  one  half  the  instrumental 
power  or  acuteness  of  sense  as  to  discover  it. 

"Mr.  Lockyer,  immediately  after  his  suggestion 
was  published,  had  set  about  procuring  a  suitable 
instrument,  and  was  assisted  by  a  grant  from  the 
treasury  of  the  Royal  Society.  After  a  long  delay, 
consequent  in  part  upon  the  death  of  the  optician 
who  had  first  undertaken  its  construction,  and  partly 
due  to  other  causes,  he  received  the  new  spectroscope 
just  as  the  report  of  Herschel's  and  Tennant's  ob- 
servations reached  England.  Hastily  adjusting  the 
instrument,  not  yet  entirely  completed,  he  at  once 
applied  it  to  his  telescope,  and  without  difficulty 
found  the  lines,  and  verified  their  position.  He  imme- 
diately also  discovered  them  to  be  visible  around  the 
whole  circumference  of  the  sun,  and  consequently 

141 


THE  SUN 

that  the  protuberances  are  mere  extensions  of  a  con- 
tinuous solar  envelope,  to  which,  as  mentioned  above, 
was  given  the  name  of  Chromosphere.  (He  does  not 
se'em  to  have  been  aware  of  the  earlier  and  similar 
conclusions  of  Arago,  Grant,  Secchi,  and  others.) 
He  at  once  communicated  his  results  to  the  Royal 
Society,  and  also  to  the  French  Academy  of  Sciences, 
and,  by  one  of  the  curious  coincidences  which  so 
frequently  occur,  his  letter  and  Janssen's  were  read 
at  the  same  meeting,  and  within  a  few  minutes  of 
each  other. 

"The  discovery  excited  the  greatest  enthusiasm, 
and  in  1872  the  French  Government  struck  a  gold 
medal  in  honor  of  the  two  astronomers,  bearing  their 
united  effigies. 

"  It  immediately  occurred  to  several  observers, 
Janssen,  Lockyer,  Zollner,  and  others,  that  by  giving 
a  rapid  motion  of  vibration  or  rotation  to  the  slit  of 
the  spectroscope  it  would  be  possible  to  perceive  the 
whole  contour  and  detail  of  a  protuberance  at  once, 
but  it  seems  to  have  been  reserved  for  Mr.  Huggins 
to  be  the  first  to  show  practically  that  a  still  simpler 
device  would  answer  the  same  purpose.  With  a 
spectroscope  of  sufficient  dispersive  power  it  is  only 
necessary  to  widen  the  slit  of  the  instrument  by  the 
proper  adjusting  screw.  As  the  slit  is  widened,  more 
and  more  of  the  protuberance  becomes  visible,  and, 
if  not  too  large,  the  whole  can  be  seen  at  once :  with 
the  widening  of  the  slit,  however,  the  brightness  of 
the  background  increases,  so  that  the  finer  details  of 

143 


ECLIPSES  AND  SOLAR  ENVELOPES 


FIG.  27. — HUGGINS'S  FIRST  OBSERVATION 
OF  A  PROMINENCE  IN  FULL  SUNSHINE. 


the  object  are/less  clearly  seen,  and  a  limit  is  soon 
reached  beyond  which  further  widening  is  disadvan- 
tageous. /The  higher  the  dispersive  power  of  the 
spectroscope  the  wider  the  slit  that  can  be  used,  and 
the  larger  the  protuberance  that  can  be  examined  as  a 
whole — within  certain  limits,  however.  It  is  not 
difficult  with  our  latest  spectroscopes,  diffraction 
instruments  espe- 
cially, to  reach  a 
dispersion  so  great 
that  even  the  C 
line  becomes  broad 
and  hazy,  like  the 
b  lines  in  an  ordi- 
nary instrument. 
In  that  case  each 
luminous  point  in  the  prominence  itself  is  represented 
in  the  image  of  the  prominence,  not  by  a  point,  as  it 
should  be  to  give  clear  definition,  but  by  a  streak  at 
right  angles  to  the  spectrum  lines. 

Spectrum  of  the  Chromosphere  and  Prominences. 

11  The  spectra  of  the  chromosphere  and  prominences 
are  very  interesting  in  their  relations  to  that  of  the 
photosphere,  and  present  many  peculiarities  which 
are  not  yet  fully  explained.  At  times  and  in  places 
where  some  special  disturbance  is  going  on — fre- 
quently in  the  neighborhood  of  spots  at  the  time* 
when  they  are  just  passing  around  the  limb  of  the 
disk — the  spectrum,  at  the  base  of  the  chromosphere^ 

143 


THE  SUN 

is  very  complicated,  consisting  of  hundreds  of  bright 
lines.  In  the  course  of  a  few  weeks  of  observation  at 
Sherman  in  1872,  the  writer  made  out  a  list  of  two 
hundred  and  seventy-three,  and  more  recent  observa- 
tions have  added  largely  to  the  number — at  least 
fifty  lines  within  the  limits  of  the  visible  spectrum, 
and,  by  photography,  at  least  eighty  in  the  ultra- 
violet. The  majority  of  the  lines,  however,  are  seen 
only  occasionally,  for  a  few  minutes  at  a  time,  when 
the  gases  and  vapors,  which  generally  lie  low,  mainly 
in  the  interstices  of  the  clouds  which  constitute  the 
photosphere,  and  below  its  upper  surface,  are  ele- 
vated for  the  time  being  by  some  eruptive  action. 
For  the  most  part,  the  lines  which  appear  only  at  such 
times  are  simply  " reversals"  of  the  more  prominent 
dark  lines  of  the  ordinary  solar  spectrum.  But  the  se- 
lection of  the  lines  seems  most  capricious;  one  is 
taken,  and  another  left,  though  belonging  to  the  same 
element,  of  equal  intensity,  and  close  beside  the  first. 
It  is  evident  that  the  subject  needs  a  detailed  and 
careful  study,  combining  solar  observations  with 
laboratory- work  upon  the  spectra  of  the  elements 
concerned,  before  a  satisfactory  account  can  be  given 
of  all  the  peculiar  behavior  observed. 

"The  lines  composing  the  true  chromosphere  spec- 
trum, if  we  may  call  it  so  (that  is,  those  which  are 
always  observable  in  it  with  suitable  appliances),  are 
not  very  numerous,  and  we  give  the  following  list, 
designating  them  by  their  wave  length,  as  given  by 
Rowland : 

144 


ECLIPSES  AND   SOLAR   ENVELOPES 


1. 

7065.50. 

Helium. 

2. 

6563.05,     C. 

Hydrogen  (Ha). 

3. 

5875.98,  D3.  (close  double)  . 

Helium. 

4. 

5316.87. 

5. 

4861.50,     F. 

Hydrogen  (HJ8). 

6. 

4471.80,    /. 

Helium. 

7. 

4340.66,     g  (near  G). 

Hydrogen  (Hy). 

8. 

4101.85,     h. 

Hydrogen  (H8). 

9. 

3970.20     (inH). 

Hydrogen  (He). 

10. 

3968.56,  H. 

Calcium. 

11. 

3933.86,  K. 

Calcium. 

"The  first  line  is  generally  very  difficult  to  see, 
though  sometimes  pretty  conspicuous.  It  is  in  the 
led,  between  B  and  a,  and  has  a  very  faint  corre- 
sponding dark  line.  No.  3  has  no  dark  line  corre- 
sponding as  a  usual  thing,  though  occasionally  one 
appears,  especially  in  the  neighborhood  of  sun  spots. 
No.  9  is  quite  within  the  broad  shade  of  the  H-line, 
which  thus  appears  double  in  the  chromosphere  spec- 
trum. 

"The  eleven  lines  mentioned  above  are  invariably 
present  in  the  spectrum  of  the  chromosphere ;  a  much 
larger  number  make  their  appearance  on  very  slight 
provocation.  They  are: 


1'. 

6678.2. 

Helium. 

11'. 

5183.8,  bi. 

Magnesium. 

2'. 

6431.1. 

Iron. 

12'. 

5172.9,62. 

Magnesium. 

3'. 

6141.9. 

Barium. 

13'. 

5169.2,  63. 

Iron. 

4'. 

5896.2, 

Di.  Sodium. 

14'. 

5167.6,64. 

Magnesium. 

5'. 

5890.2, 

D2.  Sodium. 

15'. 

5018.6. 

Iron. 

6'. 

5363.0. 

Iron.     ? 

16'. 

5015.8. 

Helium. 

7'. 

5284.6. 

Titanium?     ? 

17'. 

4934.3. 

Barium. 

8'. 

5276.2. 

Chromium.     ? 

18'. 

4924.1. 

Iron. 

9'. 

5234.7. 

Manganese. 

19'. 

4922.3. 

Helium. 

10'. 

5198.2. 

?     ? 

20'. 

4919.1. 

Iron.     ? 

145 


THE  SUN 


21'. 

4900.3. 

Barium. 

28'. 

4236.1. 

Iron. 

22'. 

4584.1. 

Iron. 

29'. 

4233.8. 

Iron. 

23'. 

4501.4. 

Titanium. 

30'. 

4226.9. 

Calcium. 

24'. 

4491.5. 

Manganese. 

31'. 

4215.7. 

Stront.am. 

25'. 

4490.2. 

Manganese. 

32'. 

4077.9. 

Strontium. 

26'. 

4469.5. 

Iron. 

33'. 

4026.0. 

Helium. 

27'. 

4245.5. 

Iron.                         34'. 

3889.1. 

Hydrogen  (HO 

"It  is  not  intended,  however,  to  intimate  that,  if 
one  of  these  appears,  all  of  them  will  do  so,  nor  that 
they  are  equally  conspicuous  or  equally  common.  To 
a  certain  degree,  also,  their  selection  by  the  writer  is 
arbitrary,  for  there  are  nearly  as  many  more  which 
are  seen  pretty  frequently,  and  some  of  them  may 
very  possibly.be  found  hereafter  to  deserve  a  place 
upon  the  list  rather  than  some  that  have  been  in- 
cluded. 

"It  requires  careful  manipulation  to  bring  out  the 
fainter  and  finer  lines  satisfactorily.  The  slit  must 
be  adjusted  with  extreme  care  to  the  focal  plane  of 
the  rays  under  examination,  placed  tangential  to  the 
solar  image,  and  brought  exactly  to  the  edge  of  the 
disk.  A  thousandth  of  an  inch  in  its  position  will 
often  make  the  whole  difference  between  a  successful 
operation  and  its  failure,  and  even  a  slight  unsteadi- 
ness of  the  air  will  diminish  the  number  of  bright 
lines  visible  by  at  least  one  half. 

"As  the  majority  of  the  lines  are  developed  only  by 
more  or  less  unusual  disturbances  of  the  solar  surface, 
it  naturally  happens  that  one  very  often  finds  them 
distorted  or  displaced  by  the  motions  of  the  gases 
along  the  line  of  sight  (toward  or  from  the  observer) , 

146 


ECLIPSES  AND  SOLAR  ENVELOPES 


as  explained  in  a  previous  chapter,  producing  what 
Lockyer  calls  "  motion-forms. "  Occasionally,  also, 
we  meet  with  "  double  reversals, "  so  called,  especially 
in  the  lines  of  magnesium  and  sodium.  The  (dark) 
lines  of  these  substances  are  rather  wide  in  the  solar 


in    the    chromosphere 


spectrum.     When    reversed 

spectrum,  the  phenomenon 

usually  consists  of  a  thin 

bright  line  down  the  center 

of  the  wider  dark  band :  in 

a  double  reversal  the  bright 

line  widens  and  a  fine  dark 

line  appears  in  its  center,  so 

that  we  have  a  central  dark 

line,  a  bright  one  on  each   FlG-  ^rDouB™ 

THE  D-LINES.     (October, 

side  of  it,  and  outside  of  the 

bright  lines  a  dark  shade  on  both  sides.  Fig.  28 
represents  such  a  double  reversal  of  the  D-lines 
observed  by  the  writer  on  several  occasions  in 
1880.  The  phenomenon  seems  to  be  due  to  the  pres- 
ence of  an  unusual  quantity  of  the  vapor  at  a  consid- 
erable density,  and  is  the  precise  correlative  of  what 
is  sometimes  seen  in  the  spectrum  of  a  sodium-flame. 
The  two  D-lines  of  sodium  each  becomes  itself  double, 
so  that  we  get  pairs  of  bright  lines  in  place  of  single 
lines.  The  electric  arc  often  shows  this  still  more 
finely. 

"At  the  base  of  a  prominence,  the  C,  F,  H,  and  K 
lines  are  always  thus  doubly  reversed.  Fig.  29  is  from 
a  recent  photograph  of  the  C-line  obtained  at  Prince- 

147 


THE  SUN 


ton,  by  Mr.  Reed,  with  the  large  telescope  and  spec- 
troscope.    The  slit  was  tangential  to  the  sun's  limb. 

Of  course,  an 
isochromatic 
plate  and  a 
long  exposure 
were  required 
to  get  such 
an  impression 
from  the 
"ruby  light" 
of  that  part 
of  the  spec- 
trum. When 
the  slit  is  ad- 
justed to  cross 
the  sun's  limb  radially  the  bright  lines  where  they 
project  beyond  the  spectrum  of  the  photosphere 
assume  the  "  arrow-headed "  form  shown  in  Fig.  30. 
"  Generally  speaking,  the  spectrum  of  a  prominence 
is  simpler  than  that  of  the  chromosphere  at  its  base. 
We  seldom  find  any 
lines  except  C,  D3,  F, 
g,  h,  H  and  K,  at  a 
considerable  elevation 
above  the  photosphere, 
though  /  is  sometimes 
met  with.  On  rare 
occasions,  also,  the  vapors  of  sodium  and  magnesium 
are  carried  into  the  higher  regions,  and  once  or  twice 

148 


FIG.  29.— DOUBLE   REVERSAL    or 
(Photographed.) 


FIG.  30. 


ECLIPSES  AND  SOLAR  ENVELOPES 

the  writer  has  seen  the  line  No.  1  of  the  second  list 
(6678.2)  in  the  upper  portions  of  a  prominence. 

Observation  of  Prominences. 

"  When  the  spectroscope  is  used  as  a  means  of  ren- 
dering visible  the  forms  and  features  of  the  promi- 
nences, the  only  difference  is  that  the  slit  is  more  or 
less  widened. 

"The  telescope  is  directed  so  that  the  solar  image 
shall  fall  with  that  portion  of  its  limb  which  is  to  be 
examined  just  tangent  to  the 
opened  slit,  as  in  Fig.  31, 
which  represents  the  slit-plate 
of  the  spectroscope,  with  the 
image  of  the  sun  in  position 
for  observation. 

"If,  now,  a  prominence  ex-    Fl«-  si.— OPENED  SLIT  OP 

...  -      .  ,  THE  SPECTROSCOPE. 

ists  at  this  part  of  the  sun  s 

limb  (as  would  probably  be  the  case,  considering  the 
proximity  of  the  spot  shown  in  the  figure),  and  if 
the  spectroscope  itself  is  so  adjusted  that  the  C-line 
falls  in  the  center  of  the  field  of  view,  then,  on  looking 
into  the  eyepiece,  one  will  see  something  much  like 
Fig.  32.  The  red  portion  of  the  spectrum  will  stretch 
athwart  the  field  of  view  like  a  scarlet  ribbon,  with  a 
darkish  band  across  it,  and  in  that  band  will  appear 
the  prominences,  like  scarlet  clouds — so  like  our  own 
terrestrial  clouds,  indeed,  in  form  and  texture,  that 
the  resemblance  is  quite  startling :  one  might  almost 
think  he  was  looking  out  through  a  partly  opened 

149 


THE  SUN 

door  upon  a  sunset  sky,  except  that  there  is  no  variety 
or  contrast  of  color;  all  the  cloudlets  are  of  the  same 
pure  scarlet  hue.  Along  the  edge  of  the  opening  is 
seen  the  chromosphere,  more  brilliant  than  the  clouds 
which  rise  from  it  or  float  above  it,  and  for  the  most 
part  made  up  of  minute  tongues  and  filaments.  Usu- 
ally, however,  the  definition  of  the  chromosphere  is 
less  distinct  than  that  of  the  higher  clouds.  The 
reason  is,  that  close  to  the  limb  of  the  sun,  where  the 
temperature  and  pressure  are  highest,  the  hydrogen 
is  in  such  a  state  that  the  lines  of  its  spectrum  are 
widened  and  "  winged, "  something  like  those  of  mag- 
nesium, though  to  a  less  extent.  Each  point  in  the 
chromosphere,  therefore,  when  viewed  through  the 
opened  slit,  appears  not  as  a  point,  but  as  a  short  line, 
directed  lengthwise  in  the  spectrum.  As  the  length 
of  this  line  depends  upon  the  dispersive  power  of  the 
spectroscope,  it  is  easy  to  see  that  it  is  possible  to  go 
too  far  in  this  respect.  The  lower  the  dispersion  the 
more  distinct  the  image  obtained,  but  also  the  fainter 
as  compared  with  the  background  upon  which  it  is 
seen. 

"Just  beneath  the  chromosphere  (at  a  in  the  cut) 
the  appearance  is  as  if  the  edge  of  the  sun  was  dark,  a 
phenomenon  which  for  some  time  was  very  puzzling. 
Its  explanation  lies  in  the  " double  reversal"  of  the 
C-line  at  the  base  of  the  chromosphere,  discussed  and 
figured  a  few  pages  back. 

"If  the  spectroscope  is  adjusted  upon  the  F-line, 
instead  of  C,  then  a  similar  image  of  the  prominences 

150 


ECLIPSES  AND   SOLAR  ENVELOPES 


and  chromosphere  is  seen,  only  blue  instead  of  scarlet ; 
usually,  however,  since  the  F-line  is  hazier  and  more 
winged  than  C,  this  blue  image  is  somewhat  less  per- 
fect in  its  details  and  definition,  and  is  therefore  less 
used  for  observation.  Similar  effects  are  obtained  by 
means  of  the  yellow  line  near  D,  and  the  violet  line  near 
G.  With  suitable  precautions,  using  a  violet  shade- 
glass  before  the 
eye,  and  carefully 
shutting  out  all 
extraneous  light, 
the  H  and  K  lines 
can  also  be  used; 
but  visual  observa- 
tions in  this  part 
of  the  spectrum  are 
extremely  difficult 
and  unsatisfactory. 

"With      photog-    FlG-  32- — CHROMOSPHERE  AND  PROMINENCES 

,  .  ,  .  AS   SEEN    IN  THE   SPECTRUM. 

raphy  the  case  is 

the  reverse — these  lines  are  then  precisely  those 
which  can  be  employed  most  easily  and  conveniently. 
We  shall  recur  to  this  a  little  later. 

"  Professor  Wirilock  and  Mr.  Lockyer  have  at- 
tempted, by  using  an  annular  opening  instead  of  the 
ordinary  slit,  to  obtain  a  view  of  the  whole  circum- 
ference of  the  sun  at  once,  and  have  succeeded.  With 
a  spectroscope  of  sufficient  power,  and  adjustments 
delicate  enough,  the  thing  can  be  done;  but  as  yet  no 
very  satisfactory  results  appear  to  have  been  reached. 
12  151 


THE  SUN 

We  still  (in  visual  observations)  have  to  examine  the 
circumference  piecemeal,  so  to  speak,  readjusting  the 
instrument  at  each  point,  to  make  the  slit  tangen- 
tial to  the  limb. 

"The  number  of  protuberances  of  considerable 
magnitude  (exceeding  ten  thousand  miles  in  altitude) , 
visible  at  any  one  time  on  the  circumference  of  the 
sun,  is  never  very  great,  rarely  reaching  twenty-five 
or  thirty.  Their  number,  however,  varies  extremely 
with  the  number  of  sun  spots:  during  a  sun-spot 
minimum  there  are  not  unfrequently  occasions  when 
not  a  single  one  can  be  found,  though  even  during 
those  years  the  more  usual  number  is  five  or  six — 
some  of  which  often  are  of  considerable  size.  The 
observations  of  Tacchini  and  Secchi  have  showed  that 
their  numbers  closely  follow  the  march  of  the  sun 
spots  though  never  falling  quite  so  low. 

"To  Tacchini  we  owe  our  most  complete  record  of 
these  objects,  now  continuous  since  1872,  giving  their 
number  and  distribution  upon  the  sun,  with  drawings 
of  all  that  were  specially  remarkable'.  Many  others 
have  cooperated  in  observations  of  this  kind:  the 
Hungarian  observers,  Fenyi  at  Kalocsa,  and  Von 
Gothard  at  Hereny,  have  given  us  many  fine  descrip- 
tions and  delineations.  Father  Perry  and  his  assis- 
tant Sidgreaves,  at  Stonyhurst,  also  deserve  a  special 
mention. 

"Their  distribution  on  the  sun's  surface  is  in  some 
respects  similar  to  that  of  the  spots,  but  with  impor- 
tant differences.  The  spots  are  confined  within  40° 

152 


ECLIPSES  AND   SOLAR   ENVELOPES 

of  the  sun's  equator,  being  most  numerous  at  a 
solar  latitude  of  about  20°  on  each  hemisphere. 
Now,  the  protuberances  are  most  numerous  pre- 
cisely where  the  spots  are  most  abundant,  but 
they  do  not  disappear  at  a  latitude  of  40°;  they 
are  found  even  at  the  poles,  and  from  the  latitude 


TW 

C- 

1414 

2S'\  i      5 

Protuberances      Jut-—"" 
2767                lA_jy  

1853   61 

1871                  Pcj—  4-**£__ 

FIG.  33. — RELATIVE  FREQUENCY  OF  PROTUBERANCES  AND  SUN-SPOTS. 

of  60°   actually  increase  in  number  to   a   latitude 
of  about  75°. 

"The  annexed  diagram,  Fig.  33,  represents  the 
relative  frequency  of  the  protuberances  and  spots  on 
the  different  portions  of  the  solar  surface.  On  the 
left  side  is  given  the  result  of  Carrington's  observa- 
tion of  1,386  spots  between  1853  and  1861,  and  on 
the  right  the  result  of  Secchi's  observation's  of 

153 


THE  SUN 

2,767  l  protuberances  in  1871.  The  length  of  each  ra- 
dial line  represents  the  number  of  spots  or  protuber- 
ances observed  at  each  particular  latitude  on  a  scale 
of  a  quarter  of  an  inch  to  the  hundred ;  for  example, 
Secchi  gives  228  protuberances  as  the  number  ob- 
served during  the  period  of  his  work  between  10°  and 
20°  of  south  latitude,  and  the  corresponding  line 
drawn  at  15°  south,  on  the  left-hand  side  of  the  figure 
is  therefore  made  Iff  or  .57  of  an  inch  long.  The 
other  lines  are  laid  off  in  the  same  way,  and  thus  the 
irregular  curve  drawn  through  their  extremities  rep- 
resents to  the  eye  the  relative  frequency  of  these  phe- 
nomena in  the  different  solar  latitudes.  The  dotted 
line  on  the  right-hand  side  represents  in  the  same 

manner  and  on  the  same  scale  the  distribution  of  the 

\ 

larger  protuberances,  having  an  altitude  of  more  than 
1',  or  27,000  miles. 

"A  mere  inspection  of  the  diagram  shows  at  once 
that,  while  the  prominences  may,  and  in  fact  often 
do,  have  a  close  connection  with  the  spots,  they  are 
yet  to  some  extent  independent  phenomena. 

"A  careful  study  of  the  subject  shows  that  they 
are  much  more  closely  related  to  the  faculse.2  In 
many  cases,  at  least,  facula?,  when  followed  to  the 


1  The  2,767  prominences  are  not  all  different  ones.     If  any  of  the 
prominences  observed  on  one  day  remained  visible  the  next,  they 
were  recorded  afresh;  and,  as  a  prominence  near  the  pole  would  be 
carried  but  slowly  out  of  sight  by  the  sun's  rotation,  it  is  thus  easy 
to  see  how  the  number  of  prominences  recorded  in  the  polar  regions 
is  so  large. 

2  See  page  109  [of  Young]. 

154 


ECLIPSES   AND   SOLAR   ENVELOPES 

limb  of  the  sun,  have  been  found  to  be  surrounded  by 
prominences,  and  there  is  reason  to  suppose  that  the 
fact  is  a  general  one.  The  spots,  on  the  other  hand, 
when  they  reach  the  border  of  the  sun's  image,  are 
commonly  surrounded  by  prominences  more  or  less 
completely,  but  seldom  overlaid  by  them.  Indeed, 
Respighi  asserts  (and  the  most  careful  observations 
we  have  been  able  to  make  confirm  his  statement) 
that  as  a  general  rule  the  chromosphere  is  consider- 
ably depressed  immediately  over  a  spot.  Secchi,  how- 
ever, denies  this. 

Magnitude  and  Classification  of  Prominences. 

"The  protuberances  differ  greatly  in  magnitude. 
.The  average  depth  of  the  chromosphere  is  not  far 
from  10"  or  12",  or  about  5,000  or  6,000  miles,  and  it 
is  not,  therefore,  customary  to  note  as  a  prominence 
any  cloud  with  an  elevation  of  less  than  15"  or  20"- 
7,000  to  9,000  miles.  Of  the  2,767  already  quoted, 
1,964  attained  an  altitude  of  40",  or  18,000  miles,  and 
it  is  worthy  of  notice  that  the  smaller  ones  are  so  few, 
only  about  one  third  of  the  whole:  751,  or  nearly  one 
fourth  of  the  whole,  reached  a  height  of  over  1',  or 
28,000  miles;  the  precise  number  which  reached 
greater  elevations  is  not  mentioned,  but  several  ex- 
ceeded 3',  or  84,000  miles.  It  is  only  rather  rarely 
that  they  reach  elevations  as  great  as  100,000  miles. 
The  writer  has  in  all  seen,  perhaps,  three  or  four 
which  exceeded  150,000  miles,  and  Secchi  has  re- 
corded one  of  300,000  miles.  On  October  7,  .1880, 

155 


THE  SUN 

the  writer  observed  one  which  attained  the  still  un- 
equaled  height  of  over  13'  of  arc,  or  350,000  miles. 
When  first  seen,  on  the  southeast  limb  of  the  sun, 
about  10.30  A.M.,  it  was  a  "horn"  of  ordinary  appear- 
ance, some  40,000  miles  in  elevation,  and  attracted 
no  special  attention.  When  next  seen,  half  an  hour 
later,  it  had  become  very  brilliant  and  had  doubled 
its  height :  during  the  next  hour  it  stretched  upward 
until  it  reached  the  enormous  altitude  mentioned, 
breaking  up  into  filaments  which  gradually  faded 
away,  until,  by  12.30  P.M.,  there  was  nothing  left.  A 
telescopic  examination  of  the  sun's  disk  showed  noth- 
ing to  account  for  such  an  extraordinary  outburst, 
except  some  small  and  not  very  brilliant  facula?. 
While  it  was  extending  upward  most  rapidly  a  violent 
cyclonic  motion  was  shown  by  the  displacement  of 
the  spectrum  lines,  and  H  and  K  were  reversed 
through  its  whole  height. 

"In  their  form  and  structure  the  protuberances, 
differ  as  widely  as  in  their  magnitude.  Two  princi- 
pal classes  are  recognized  by  all  observers — the  qui- 
escent, cloud-formed  or  hydrogenous,  and  the  eruptive 
or  metallic.  By  Secchi  these  are  each  further  sub- 
divided into  several  sub-classes  or  varieties,  between 
which,  however,  it  is  not  always  easy  to  maintain  the 
distinctions. 

"And  here  perhaps  is  the  proper  place  to  mention 
that  Trouvelot  insists  on  the  existence  of  "dark" 
prominences — i.e.,  clouds  of  cooler  hydrogen  that 
absorb  the  light  of  the  hydrogen  behind  them;  but 

156 


ECLIPSES   AND   SOLAR   ENVELOPES 


Three  figures  of  the  same  prominence, 
seen  July  25,  1872. 


FIG.  34. 

AS  SEEN   AT  2.15   P.  M. 


FIG.  38. 
SHEAF  AND  VOLUTES. 


FIG.  35. 

AS  SEEN   AT  2.45  P.  M. 


FIG.  3<>. 

AS  SEEN  AT  3.30  P.  M. 

Scale,  100,000  miles  to  the  inch. 

ERUPTIVE  PROMINENCES 
157 


THE  SUN 

there  is  no  proof,  we  think,  that  these  are  anything 
but  "holes. "  Tacchini,  on  the  other  hand,  is  disposed 
to  assert  the  existence  of  "  white"  prominences,  which 
give  a  continuous  spectrum,  and  so  are  not  reached 
by  spectroscopic  observation,  though  conspicuous  to 
the  eye,  and  on  the  photographic  plate,  at  the  time  of 
a  total  eclipse,  as  in  1883  and  December,  1889.  But 
the  evidence  hardly  warrants  confident  belief  in  the 
existence  of  such  objects. 

"The  quiescent  prominences  in  form  and  texture 
resemble,  with  almost  perfect  exactness,  our  terres- 
trial clouds,  and  differ  among  themselves  as  much  and 
in  the  same  manner.  The  familiar  cirrus  and  stratus 
types  are  very  common,  the  former  especially,  while 
the  cumulus  and  cumulo-stratus  are  less  frequent. 
The  protuberances  of  this  class  are  often  of  enormous 
magnitude,  especially  in  their  horizontal  extent  (but 
the  highest  elevations  are  attained  by  those  of  the 
eruptive  order),  and  are  comparatively  permanent, 
remaining  often  for  hours  and  days  without  serious 
change;  near  the  poles  they  sometimes  persist 
through  a  whole  solar  revolution  of  twenty- seven 
days.  Sometimes  they  appear  to  lie  upon  the  limb 
of  the  sun  like  a  bank  of  clouds  in  the  horizon;  prob- 
ably because  they  are  so  far  from  the  edge  of  the  disk 
that  only  their  upper  portions  are  in  sight.  When 
seen  in  their  full  extent  they  are  ordinarily  connected 
to  the  underlying  chromosphere  by  slender  columns, 
which  are  usually  smallest  at  the  base,  and  appear 
often  to  be  made  up  of  separate  filaments  closely  in- 

158 


ECLIPSES   AND   SOLAR   ENVELOPES 


FIG.  40. 
CLOUDS. 


FIG.  43. 
DIFFUSE. 


FIG.  41. 
FILAMENTARY. 


FIG.  44. 
STEMMED. 


FIG.  42. 
PLUMES. 


FIG.  45. 
HORNS. 


QUIESCENT  PROMINENCES. 

Scale,  7o,000  miles  to  the  inch. 

159 


THE  SUN 

tertwined,  and  expanding  upward.  Sometimes  the 
whole  under  surface  is  fringed  with  down-hanging 
filaments,  which  remind  one  of  a  summer  shower  fall- 
ing from  a  heavy  thundercloud.  Sometimes  they 
float  entirely  free  from  the  chromosphere;  indeed,  as 
a  general  rule,  the  layer  clouds  are  attended  by  de- 
tached cloudlets  for  the  most  part  horizontal  in  their 
arrangement. 

11  The  figures  give  an  idea  of  some  of  the  general  ap- 
pearances of  this  class  of  prominences,  but  their  del- 
icate, filmy  beauty  can  be  adequately  rendered  only 
by  a  far  more  elaborate  style  of  engraving. 

"Their  spectrum  is  usually  very  simple,  consisting 
of  the  four  lines  of  hydrogen,  and  the  three  of  helium, 
with  H  and  K.  Occasionally  the  sodium  and  mag- 
nesium lines  also  appear,  and  that  even  near  the  sum- 
mit of  the  clouds;  and  this  phenomenon  was  so  much 
more  frequently  observed  in  the  clear  atmosphere  of 
Sherman  as  to  suggest  that,  if  the  power  of  our  spec- 
troscopes were  sufficiently  increased,  it  would  cease 
to  be  unusual. 

"The  genesis  of  this  sort  of  prominence  is  problem- 
atical. They  have  been  commonly  looked  upon  as 
the  debris  and  relics  of  eruptions,  consisting  of  gases 
which  have  been  ejected  from  beneath  the  solar  sur- 
face, and  then  abandoned  to  the  action  of  the  cur- 
rents of  the  sun's  upper  atmosphere.  But  near  the 
poles  of  the  sun  distinctively  eruptive  prominences 
never  appear,  and  there  is  no  evidence  of  aerial  cur- 
rents which  would  transport  to  those  regions  matters 

160 


ECLIPSES   AND   SOLAR   ENVELOPES 

ejected  nearer  the  sun's  equator.  Indeed,  the  whole 
appearance  of  these  objects  indicates  that  they  orig- 
inate where  we  see  them.  Possibly,  although  in  the 
polar  regions  there  are  no  violent  eruptions,  there 
yet  may  be  a  quiet  outpouring  of  heated  hydrogen 
sufficient  to  account  for  their  production — an  out- 
rush  issuing  through  the  smaller  pores  of  the  solar 
surface,  which  abound  near  the  poles  as  well  as 
elsewhere. 

"But  Secchi  reports  an  observation  which,  if  cor- 
rect, puts  a  very  different  face  upon  the  matter.1 
He  has  seen  isolated  cloudlets  form  and  grow  spon- 
taneously without  any  perceptible  connection  with 
the  chromosphere  or  other  masses  of  hydrogen,  just 
as  in  our  own  atmosphere  clouds  form  from  aqueous 
vapor,  already  present  in  the  air,  but  invisible  until 
some  local  cooling  or  change  of  pressure  causes  its 
condensation.  These  prominences  are,  therefore, 
formed  by  some  local  heating  or  other  luminous  ex- 
citement of  hydrogen  already  present,  and  not  by  any 

xOn  October  13,  1880,  the  writer  for  the  first  time  met  with  the 
same  phenomenon.  A  small,  bright  cloud  appeared  on  that  day, 
about  11  A.  M.,  at  an  elevation  of  some  2^'  (67,500  miles)  above  the 
limb,  without  any  evident  cause  or  any  visible  connection  with  the 
chromosphere  below.  It  grew  rapidly  without  any  sensible  rising  or 
falling,  and  in  an  hour  developed  into  a  large  stratiform  cloud, 
irregular  on  the  upper  surface,  but  nearly  flat  beneath.  From  this 
lower  surface  pendent  filaments  grew  out,  and  by  the  middle  of  the 
afternoon  the  object  had  become  one  of  the  ordinary  stemmed 
prominences,  much  like  Fig.  44. 

But  obviously  the  thing  is  very  unusual,  for  in  more  than  twenty 
years  of  observation  I  have  encountered  the  phenomenon  only  three 
times. 

161 


THE  SUN 

transportation  and  aggregation  of  materials  from  a 
distance.  The  precise  nature  of  the  action  which 
produces  this  effect  it  would  not  be  possible  to  assign 
at  present ;  but  it  is  worthy  of  note  that  the  spectro- 
scopic  observations  made  during  eclipses  rather  favor 
this  view,  by  showing  that  hydrogen,  in  a  feebly 
luminous  condition,  is  found  all  around  the  sun,  and 
at  a  very  great  altitude — far  above  the  ordinary  range 
of  prominences. 

"  Indeed,  in  most  cases  the  forms  and  changes  of 
this  class  of  prominences  so  closely  resemble  our  own 
terrestrial  clouds  that  one  is  almost  forced  to  believe 
that  they  are  surrounded  by,  and  float  in,  a  medium 
which  does  not  greatly  differ  from  themselves  in  den- 
sity, though  it  is  not  visible  in  the  spectroscopic  mode 
of  observation. 

Eruptive  Prominences. 

"The  eruptive  prominences  are  very  different- 
much  more  brilliant  and  much  more  vivacious  and 
interesting.  They  consist  usually  of  brilliant  spikes 
or  jets,  which  change  their  form  and  brightness  very 
rapidly.  For  the  most  part  they  attain  altitudes  of 
not  more  than  20,000  or  30,000  miles,  but  occasion- 
ally they  rise  far  higher  than  even  the  largest  of  the 
clouds  of  the  preceding  class.  Their  spectrum  is  very 
complicated,  especially  near  their  base,  and  often 
filled  with  bright  lines,  those  of  sodium,  magnesium, 
barium,  iron,  and  titanium,  being  especially  conspicu- 
ous, while  calcium,  chromium  manganese,  and  prob- 

162 


ECLIPSES  AND  SOLAR  ENVELOPES 


FIG.  49. 

PROMINENCE  AS  IT  APPEARED  AT  HALF- 
PAST  TWELVE  O'CLOCK,  SEPTEMBER 
7,  1871. 


FIG.  46. 
VERTICAL  FILAMENTS. 


FIG.  47. 
CYCLONE. 


FIG.  50. 

As  THE  ABOVE  APPEARED  HALF  AN  HOUR 
LATER  WHEN  THE  UP-RUSHING  HYDROGEN 
ATTAINED  A  HEIGHT  OF  MORE  THAN  200,- 
000  MILES. 


FIG.  51. 

SPOT  NEAR  THE   SUN'S  LlMB,  WITH  ACCOM- 
PANYING JETS   OF  HYDROGEN,   AS  SEEN 
FLAMES.  OCTOBER  5,  1871. 

Scale,  75,000  miles  to  the  inch. 
163 


THE  SUN 

ably  sulphur,  are  by  no  means  rare,  and  for  this 
reason  Seechi  calls  them  metallic  prominences. 

"They  usually  appear  in  the  immediate  neighbor- 
hood of  a  spot,  never  occurring  very  near  the  solar 
poles.  Their  form  and  appearance  change  with  great 
rapidity,  so  that  the  motion  can  almost  be  seen  with 
the  eye — an  interval  of  fifteen  or  twenty  minutes 
being  often  sufficient  to  transform,  quite  beyond  rec- 
ognition, a  mass  of  these  flames  fifty  thousand  miles 
high,  and  sometimes  embracing  the  whole  period  of 
their  complete  development  or  disappearance.  Some- 
times they  consist  of  pointed  rays,  diverging  in  all 
directions,  like  hedgehog-spines.  Sometimes  they 
look  like  flames;  sometimes  like  sheaves  of  grain; 
sometimes  like  whirling  waterspouts,  capped  with  a 
great  cloud;  occasionally  they  present  most  exactly 
the  appearance  of  jets  of  liquid  fire,  rising  and  falling 
in  graceful  parabolas;  frequently  they  carry  on  their 
edges  spirals  like  the  volutes  of  an  Ionic  column;  and 
continually  they  detach  filaments  which  rise  to  a 
great  elevation,  gradually  expanding  and  growing 
fainter  as  they  ascend,  until  the  eye  loses  them.  Our 
figures  present  some  of  the  more  common  and  typical 
forms,  and  illustrate  their  rapidity  of  change,  but 
there  is  no  end  to  the  number  of  curious  and  interest- 
ing appearances  which  they  exhibit  under  varying 
circumstances. 

"The  velocity  of  the  motions  often  exceeds  a  hun- 
dred miles  a  second,  and  sometimes,  though  very 
rarely,  reaches  two  hundred  miles.  That  we  have  to 

164 


ECLIPSES  AND  SOLAR  ENVELOPES 

do  with  actual  motions,  and  not  with  mere  change  of 
place  of  a  luminous  form,  is  rendered  certain  by  the 
fact  that  the  lines  of  the  spectrum  are  often  displaced 
and  distorted  in  a  manner  to  indicate  that  some  of  the 
cloud-masses  are  moving  either  toward  or  from  the 
earth  (and,  of  course,  tangential  to  the  solar  surface) 
with  similar  swiftness. 

"Fig.  52  is  a  representation  of  a  portion  of  the  spec- 
trum of  a  prominence  observed  at  Sherman  on  August 


FIG.  52. 

3,  1872,  an  observation  to  which  allusion  was  made  in 
the  preceding  chapter.  The  F-line,  at  208  of  the 
scale,  must  be  imagined  as  blazingly  brilliant,  and 
fainter  bright  lines  appear  at  203.2,  208.8,  209.4,  and 
212.1  (the  scale  is  KirchhofFs),  while  two  bands  of 
continuous  spectrum,  produced  probably  by  the  com- 
pression of  the  gas  at  the  points  of  maximum  dis- 
turbance, run  the  whole  length  of  the  figure.  At  the 

165 


THE  SUN 

upper  point  of  disturbance  F  is  drawn  out  into  a  point 
reaching  to  207.4  of  the  scale,  and  indicating  a  veloc- 
ity of  230  miles  a  second  away  from  us;  at  the  lower 
point  it  extends  to  208.7,  and  indicates  a  velocity  of 
about  250  miles  per  second  toward  us.  It  was  very 
noticeable  that  this  swift  motion  of  the  hydrogen  did 
not  seem  to  carry  with  it  many  other  substances 
which  were  at  the  time  represented  in  the  spectrum 
by  their  bright  lines;  magnesium  and  sodium  were 
somewhat  affected,  but  barium  and  the  unknown  ele- 
ment of  the  corona  were  not. " 

An  examination  of  the  sun's  limb  for  prominences 
is  made  on  every  fair  observing  day  at  many  observa- 
tories. At  the  Italian  observatories  of  Rome  and 
Catania  such  observations  have  been  continued  by 
Secchi,  Tacchini,  and  Ricco  for  about  forty  years.  A 
general  discussion  of  this  highly  valuable  mass  of 
observations  is  about  to  be  published. 

Prominences  and  the  Spectroheliograph. 

Since  the  introduction  of  the  spectroheliograph  the 
prominences  can  be  observed  much  more  satisfactor- 
ily than  before.  In  Plate  XIV,  Fig.  1  shows  a  large 
quiescent  prominence  photographed  by  Slocum  with 
the  Rumford  spectroheliograph  at  the  Yerkes  Ob- 
servatory. The  height  of  the  prominence  as  shown 
in  the  plate  is  1.6  minutes  of  arc,  or  69,000  kilometers. 
Slocum  states  that  this  prominence  lasted  probably 
continuously  for  at  least  fifty-five  days  in  the  spring 
of  1910,  but  Evershed  traces  it  twenty-seven  days 

.  166 


PLATE  XIV. 


FIG.  1.— 1910,  March  17.     G.  M.  T.     5h  30m.     LON.  7°.     LAT.  +  I<°TO  -18C 


FIG.  2.— 1910,  October  10.         G.  M.  T.     7h  56m  .8. 


FIG.  3.— 1910,  October  10.         G.  M.  T.     8h  6m  .4. 
SOLAR  PROMINENCES.     (Slocum.)     CALCIUM  (H)  SPECTROHELIOGRAMS. 


ECLIPSES  AND  SOLAR  ENVELOPES 

longer  still.  Its  southern  extremity  remained  nearly 
stationary  near  20°  south  latitude,  while  the  northern 
end  varied  greatly;  from  the  equator  on  March  4,  to 
25°  north  latitude  on  March  18;  then,  retiring, 
reached  10°  south  latitude  on  April  28.  At  no  time 
could  the  prominence  be  seen  projected  against  the 
sun's  disk  in  Slocum's  calcium  spectroheliographic 
observations.  He  saw  it  only  on  the  sun's  limbs. 
But  Evershed  and  Deslandres  photographed  it  re- 
peatedly; the  former  in  H2  calcium,  the  latter  in  K3 
calcium  and  Ha  hydrogen  light,  appearing  like  a  long 
cloud  upon  the  sun's  disk.  A  similar  feature  is  shown 
in  the  Ha  photograph  reproduced  in  Plate  VI  from 
Ellerman's  Mount  Wilson  observations  of  April  30, 
1908. 

Very  beautiful  eruptive  prominences  are  occa- 
sionally observed  with  the  spectroheliograph.  The 
two  lower  figures  in  Plate  XIV  show  an  uncommon- 
ly fine  quasi-eruptive  prominence  photographed  Oc- 
tober 10,  1910,  in  the  H  line  of  calcium,  by  Slocum  at 
the  Yerkes  Observatory.  Although  by  no  means  as 
active  as  some  eruptive  prominences,  this  one 
changed  rapidly,  and  there  may  be  seen  considerable 
differences  in  form  in  the  two  exposures,  separated 
by  a  time  interval  of  only  ten  minutes.  The  approxi- 
mate position  was  as  follows:  Solar  latitude  24°  to 
39°  S;  longitude  225°.  Height  2.5  minutes  of  arc,  or 
108,000  kilometers. 


13  167 


THE  SUN 

Recent  Flash-spectrum  Observations. 

Following  the  great  discovery  of  Janssen  and  Lock- 
yer  in  1868,  the  next  year  brought  the  important  dis- 
covery of  helium  in  the  sun — a  chemical  element  not 
found  on  the  earth  for  nearly  thirty  years  afterwards. 
Young  kept  up  the  pace  by  the  discovery  of  the 
" flash  spectrum"  at  the  total  solar  eclipse  of  1870. 
Setting  the  slit  of  his  spectroscope  where  the  chromo- 
sphere should  be,  and  keeping  his  eye  prepared  for 
what  he  was  about  to  witness,  he  saw,  as  the  last 
photospheric  rays  were  extinguished,  a  bright  line 
reversal  of  the  photospheric  spectrum  flash  out  to 
view.  It  was  not  till  1896  that  the  flash  spectrum 
was  photographed  by  Shackelton  with  a  prismatic 
camera. 

The  chromosphere  appears  as  a  very  thin  crescent, 
hence  its  spectrum  may  be  photographed  without 
slit  or  collimator.  The  appearance  of  such  spectra 
may  be  understood  from  Plate  XV,  Fig.  1,  taken  by 
S.  A.  Mitchell  at  the  eclipse  of  1905.  The  spectrum 
lines  are  each  represented  by  arcs  of  circles.  Where 
very  long  arcs  appear,  they  correspond  to  the  great 
lines  of  hydrogen  and  calcium.  These  elements  ex- 
tend much  higher  above  the  sun  than  others,  and 
hence  continue  in  sight  longer  as  the  moon  advances. 

It  was  feared  that  the  astigmatism,  which  makes 
the  concave  grating  a  valuable  laboratory  instrument, 
would  render  it  unfit  for  use  on  the  flash  spectrum 
without  a  slit.  A  slit  for  such  work  is  undesirable, 

168 


ECLIPSES  AND  SOLAR  ENVELOPES 

on  account  of  the  loss  of  light.  The  work  of  Mitchell 
in  1898,  who  used  for  the  photography  of  stellar  spec- 
tra a  Rowland  concave  grating  as  an  objective  grat- 
ing, without  slit,  paved  the  way  for  the  use  of  grat- 
ings at  the  time  of  an  eclipse.  They  were  first  used 
in  flash-spectrum  photography  in  1900. 

Successful  photographic  observations  of  the  flash 
spectrum  have  been  made  at  the  eclipses  of  1896, 
1898,  1900,  1901,  1905,  and  1908.  Among  the  observ- 
ers have  been  Shackelton,  Campbell,  Evershed, 
Dyson,  Jewell,  Frost,  Lord,  S.  A.  Mitchell,  Perrine, 
and  others.  The  observations  have  shown  that  the 
flash  spectrum,  or  spectrum  of  the  chromosphere,  is 
essentially  the  reversal  of  the  ordinary  Fraunhofer 
spectrum,  but  with  some  significant  differences. 
Many  of  the  weaker  Fraunhofer  lines,  of  course,  do 
not  appear.  The  lines  of  the  two  spectra  in  general 
bear  different  relative  intensities.  Taking  the  lines  of 
any  one  chemical  element  by  itself,  however,  the  rel- 
ative intensities  in  the  two  spectra  are  not  very  dif- 
ferent. Lockyer,  Evershed,  and  Dyson  find  in  gen- 
eral that  the  so-called  enhanced  or  spark  lines  are 
more  prominent  in  the  flash  spectrum  than  in  the 
photospheric  spectrum.1  The  cause  of  the  dis- 
crepancy between  the  line  intensities  in  the  spectra, 
as  a  whole,  seems  to  be  that  the  elements  of  higher 
atomic  weights  are  less  prominent  in  flash  spectra. 

Dyson,  in  discussing  the  Greenwich  observations 

1  Frost  and  Mitchell  were  inclined  to  question  that  this  is  general, 
but  Mitchell  seems  now  to  agree  that  it  is. 

169 


THE  SUN 

of  the  eclipses  of  1900, 1901,  and  1905,  gives  the  meas- 
ured positions  of  about  1200  lines,  and  identifica- 
tions of  most  of  them  with  single  lines,  or  with  blends 
of  several  lines,  found  in  Rowland's  tables.  The 
range  of  spectrum  observed  is  from  3295  Ang- 
stroms, in  the  ultra-violet,  to  5896  in  the  orange.  The 
average  deviation  of  the  positions  from  the  positions 
fixed  by  Rowland  is  0.04  Angstroms,  but  as  Dyson's 
spectra  are  prismatic  this  difference  is  exceeded  in 
the  green  and  yellow.  Dyson  found  twenty-six  strong 
lines  of  hydrogen  agreeing  excellently  in  position 
with  the  places  fixed  by  Balmer's  series  formula. 
Helium  is  also  a  prominent  element.  The  following 
are  the  chemical  elements  as  they  are  found  repre- 
sented by  their  spectral  lines: 

Very  strong:  Hydrogen,  Helium,  Magnesium, 
Calcium,  Scandium,  Titanium,  Chromium,  Stron- 
tium. 

Strong:  Manganese,  Iron,  Yttrium,  Zirconium, 
Barium,  Lanthanum,  Cerium,  Erbium,  Europeum. 

Not  very  strong:  Carbon,  Aluminum,  Vanadium, 
Neodymium. 

Very  weak:  Nickel,  Cobalt,  Lead. 

Possibly  shown:  Zinc,  Lanthanum,  Tantalum. 

Doubtful:  Silicon,  Gadolinium,  Praesodymium. 

Absent:1  Argon,  Neon,  Krypton. 

Not  well  shown  within  limits  of  spectrum :  Sodium. 

The  arc  lines  of  aluminum,  magnesium,  barium, 

1  Mitchell,  however,  inclines  to  think  these  elements  are  repre- 
sented in  the  flash  spectrum  by  weak  lines. 

170 


ECLIPSES  AND  SOLAR  ENVELOPES 

zinc,  and  lead  appear  to  be  present,  whereas  their 
enhanced,  or  spark,  lines  show  not  at  all,  or  faintly. 
In  this  Dyson  finds  these  elements,  exceptional,  for  in 
general  it  is  the  enhanced  lines  which  predominate  in 
the  flash. 

The  Heights  of  Different  Metals  in  the  Chromosphere. 
By  measuring  the  lengths  of  the  arcs  seen  as  flash 
spectrum  lines,  observers  have  estimated  the  heights 
to  which  the  elements  rise  in  the  chromosphere  above 
the  sun's  general  surface.  From  Sir  Norman  Lock- 
yer's  report  of  observations  of  the  eclipse  of  1898,  we 
have  the  following  values.  The  ordinary  chemical 
symbols  for  the  elements  are  used  for  short : 


Element 

Ca 

H 

He 

Sr 

Ca 

Mg 

Al 

Mn 

Fe 

C 

Var- 
ious 

Spectrum 

K 

Not 

4471 

4078 

4227 

U.V. 

3944 

Quar- 

Many 

Flut- 

Many 

lines  

given 

trip- 

tet 

ing 

lines 

let 

Includ- 

4027 

4216 

3962 

4031 

lines 

ing  Fe 

etc. 

arc 

lines 

Mean 

height 

seconds.  .  . 

13.3 

10 

7.5 

6.0 

4.4 

4.4 

3.2 

2.4 

3.2 

1.05 

1.05 

to  1.4 

Kilometers  . 

9700 

7200 

5400 

4300 

3200 

3200 

2300 

1800 

2300 

760 

760 

to  1000 

Jewell,1  from  observations  of  the  eclipses  of  1900 
and  1901,  estimates  the  chromospheric  heights  cor- 
responding to  separate  lines  of  various  elements.  He 
finds,  as  does  Lockyer  (See  Ca  above),  that  different 
lines  of  the  same  elements  yield  widely  different  val- 
ues. Thus,  for  calcium  his  heights  range  from  15,000 

1  Pub.  U.  S.  Naval  Observatory,  2  Series,  Vol.  IV,  Ap.  I.  - 
171 


THE  SUN 

down  to  100  miles,  and  for  titanium  from  3,500  to  100 
miles.  In  general  his  results  show  high  levels  for 
hydrogen,  helium,  parhelium,  magnesium,  sodium, 
and  ytterbium;  low  levels  for  chromium,  iron,  cobalt, 
nickel,  manganese,  yttrium,  cadmium,  zinc,  carbon 
(as  cyanogen),  and  vanadium;  contradictory  levels 
indicated  by  different  lines  for  calcium,  strontium, 
barium,  scandium,  and  titanium.  Most  lines  corre- 
spond to  heights  of  less  than  one  second  of  arc  (475 
miles,  760  kilometers).  Jewell  regards  the  chromo- 
sphere as  an  atmosphere  of  hydrogen  and  a  few  other 
permanent  gases,  rapidly  decreasing  in  density  out- 
ward, and  holding  as  temporary  constituents  other 
elements  as  products  of  eruptions  from  within,  or 
meteors  from  without. 

Frost  and  Mitchell,  from  observations  of  the  1900 
and  1901  eclipses,  respectively,  have  also  given  brief 
tables  of  the  heights  attained  by  different  elements  in 
the  chromosphere,  as  indicated  by  individual  spec- 
trum lines.  Their  results  differ  very  little  from  those 
above  mentioned.  Mitchell  states  that  the  lengths 
of  a  great  majority  of  the  lines  indicate  heights  not 
exceeding  0.5"  of  arc,  and  would  set  I"  of  arc  as  the 
average  depth  of  the  "  re  versing  layer." 

Jewell  very  pertinently  calls  attention  to  the  mi- 
nute quantities  of  substance  required  to  produce  spec- 
trum lines.  As  some  lines  require  less  producing 
substance  than  others,  this  may  cause  part  of  the  dis- 
crepancy between  the  heights  estimated  for  different 
lines  of  the  same  element. 


ECLIPSES  AND   SOLAR   ENVELOPES 

Mitchells  Observations  of  1905. 

My  friend,  Prof.  S.  A.  Mitchell,  has  kindly  fur- 
nished me,  in  advance  of  his  publication,  with  the 
following  description  of  his  apparatus,  and  of  the 
results  he  obtained  as  a  member  of  the  U.S.  Naval 
Observatory  expedition,  at  the  total  eclipse  of 
August,  1905.  His  flash  spectrum  is  believed  to  be 
the  best  which  has  ever  been  secured. 

"Mitchell  used  two  spectrographs  of  high  disper- 
sion, both  with  gratings.  The  first  was  a  six-inch 
Rowland  plane  grating  of  15,000  lines  to  the  inch, 
belonging  to  the  Naval  Observatory.  This  same  grat- 
ing had  been  used  by  him  in  Sumatra,  in  1901,  but 
in  1905  a  glass  achromatic  objective  of  five  inches 
aperture  was  used  instead  of  a  quartz  lens.  With  this 
instrument  special  attention  was  paid  to  the  red  end 
of  the  spectrum.  The  other  instrument  was  a  four- 
inch  grating,  ruled  on  a  parabolic  surface,  instead  of 
the  ordinary  spherical  concave  surface.  This  grating 
of  14,438  lines  per  inch  and  ten  feet  radius  of  curva- 
ture was  very  bright  in  the  first  order  on  one  side,  and 
in  the  estimation  of  Mr.  Jewell  it  was  one  of  the  best 
of  Rowland  gratings,  and  gave  spectra  equal  in 
brightness  to  that  obtained  by  the  ordinary  six-inch 
grating.  This  grating  belonged  to  the  Rumford  com- 
mittee, and  was  kindly  loaned  by  Professor  F.  A. 
Saunders  of  Syracuse  University. 

"Such  a  spectrograph  used  for  eclipse  work  is  of 
the  simplest  form  imaginable.  Light  from  the  coelo- 

173 


THE  SUN 

stat  mirror,  reflected  horizontally,  falls  on  the  grating, 
where  it  is  diffracted,  and  is  then  brought  to  focus  on 
the  photographic  plate,  five  feet  distant.  Grating  and 
plate  holder  are  placed  in  a  wooden  box,  and  if  the 
grating  and  photographic  plate  are  perpendicular  to 
the  diffracted  beam,  the  spectrum  is  " normal. "  As 
the  spectrum  was  brought  to  a  focus  on  a  circle  of 
thirty  inches  radius,  it  was  impossible  to  bend  the 
photographic  plate,  and  heavy  gelatine  films  were 
used.  The  spectra  were  focussed  by  using  a  col- 
limating  apparatus  consisting  of  a  slit  between  two 
concave  mirrors  (which  were  previously  adjusted  by 
the  use  of  a  five-inch  visual  telescope).  The  spectra 
were  focussed  visually,  and  test  photographs  were 
made  in  order  to  check  the  ultra-violet  focus.  The 
excellence  of  the  focus  is  shown  by  the  flash  spectra, 
which  were  photographed  in  the  first  order. 

"The  parabolic  grating  spectra  extend  from  X  3,300 
in  the  ultra-violet  to  the  D  lines  at  X  5,890  in  the 
orange.  The  plane  grating  spectrogram  continues 
in  the  red  to  the  C  line.  The  length  of  the  spectrum 
taken  with  the  former  grating  is  9.5  inches.  The  spec- 
trum is  very  nearly  normal  throughout  its  whole 
extent;  the  dispersion,  therefore,  is  such  that  one 
millimeter  is  equal  to  10.8  Angstrom  units.-  This  is  a 
dispersion  about  equal  to  that  obtained  by  the  three- 
prism  spectrographs  attached  to  the  great  Lick  or 
Yerkes  telescopes.  As  the  grating  at  the  eclipse  was 
used  as  an  objective  grating  without  slit,  it  had  a  dis- 
persion a  little  less  than  a  quarter  of  that  obtained 

174 


Q 


ECLIPSES   AND   SOLAR   ENVELOPES 

with  a  21.5  foot  grating  in  the  first  order  on  the  or- 
dinary Rowland  mounting. 

"  As  is  seen  from  the  illustration  (Plate  XV,  Fig.  1, 
where  unfortunately  a  great  amount  of  fine  detail  is 
lost  in  reproduction)  the  definition  is  excellent.  The 
extreme  ultra-violet  is  not  in  quite  so  good  a  focus  as 
the  region  from  K  to  D,  where  the  definition  is  per- 
fect. About  4,000  lines  were  measured  in  the  region 
from  X  3,300  to  X  5,900.  On  account  of  the  strong  con- 
tinuous spectrum  throughout  the  photographed  spec- 
trum, it  was  a  little  difficult  to  see  the  spectrum  lines, 
especially  when  they  were  faint.  The  spectrum  lines 
being  curved,  it  was  necessary  to  measure  at  the  same 
part  of  each  line.  Moreover,  since  no  slit  was  used,  it 
was  necessary  to  measure  the  position  of  the  line  at 
the  moon's  edge.  Evidently  the  height  above  the 
sun's  limb  of  the  metallic  vapor  forming  a  given  spec- 
trum line  has  much  to  do  with  its  appearance  on  the 
photographic  plate,  and  the  middle  of  the  measured 
line  will  not  give  its  exact  wave  length. 

"  Preliminary  wave  lengths  of  the  flash  lines  were 
directly  obtained  from  the  measures.  These  were 
compared  with  Rowland's  tables.  Each  line  from 
Rowland  which  was  identified  with  certainty  was 
taken  as  a  standard  to  obtain  adjusted  values  of  the 
flash  wave  lengths.  The  smaller  dispersion  in  the 
flash  spectrum  caused  lines  in  Rowland  to  be  blended 
together;  and  in  such  blends  it  was  difficult  to  know 
what  exact  wave  length  to  assume.  Consequently,  if 
the  flash  lines  were  identified  with  single  lines  in 

175 


THE  SUN 

Rowland's  tables,  they  were  taken  as  standards. 
Since  the  scale-value  assumed  was  only  an  approxi- 
mate one,  and  the  spectrum  was  not  strictly  normal,  a 
Least  Squares  adjustment  was  made  in  a  manner 
suggested  by  Professor  C.  Runge,  Kaiser  Wilhelm 
Professor  at  Columbia  University  in  1909-10.  As  the 
result  of  this  adjustment,  the  probable  error  of  a 
single  determination  of  a  wave  length  throughout  the 
spectrum  is  about  =«=  0.025  Angstrom  units.  The 
small  size  of  this  error  will  be  appreciated  when  one 
remembers  that  a  series  of  cusps  were  measured,  and 
that  an  error  in  measurement  of  a  thousandth  of  a 
millimeter,  or  one  micron,  corresponds  to  a  discrep- 
ancy of  0.01  A.  U. 

"Such  accurate  wave  lengths  of  the  flash  spectra 
lend  the  possibility  of  a  close  comparison  with  the 
Fraunhofer  spectrum.  Such  a  comparison  shows  with 
great  certainty  that  the  flash  spectrum  is  but  a  rever- 
sal of  the  Fraunhofer  lines.  Almost  every  line  in  the 
ordinary  solar  spectrum  with  an  intensity  of  3  or 
greater  on  Rowland's  scale  is  found  in  the  flash  spec- 
trum, in  many  cases  two  or  more  lines  being  blended 
into  one  in  the  photograph  of  smaller  dispersion. 
Though  all  the  strong  Fraunhofer  lines  are  found  in 
the  flash  spectrum,  the  converse  is  not  true;  for 
there  are  many  strong  lines  in  the  flash  spectrum 
which  have  no  equivalent  in  the  ordinary  spectrum. 
In  addition  to  this  fact,  there  is  the  further  difference 
that  there  are  remarkable  inequalities  in  intensity 
between  the  lines  of  the  two  spectra. 

17G 


ECLIPSES  AND  SOLAR  ENVELOPES 

"It  was  pointed  out  by  Evershed  that  we  could 
easily  imagine  two  separate  gases  in  the  sun's  enve- 
lope which  would  have  absorption  lines  of  the  same 
intensities,  but  whose  emission  spectra  would  differ 
very  much  in  intensities.  A  heavy  gas,  lying  in  a 
thin  layer  above  the  photosphere,  might  absorb  the 
solar  light  exactly  to  the  same  extent  as  a  less  dense 
layer  extending  to  greater  altitudes.  As  the  moon 
successively  passes  over  layers  at  the  time  of  an 
eclipse,  the  lighter  gas  would  give  lines  of  the  greater 
intensities  in  the  flash  spectrum.  As  is  well  known, 
the  helium  lines  appear  as  strong  lines  in  the  flash 
spectrum;  they  are  lacking  in  the  Fraunhofer  spec- 
trum. Over  thirty  lines  of  the  hydrogen  series  have 
been  counted  in  Mitchell's  1905  spectra.  In  Plate 
XV,  Fig.  2,  a  portion  of  the  spectrum  is  shown  greatly 
enlarged. 

"For  the  purpose  of  a  closer  comparison,  the  re- 
sults of  the  measures  of  an  extent  of  the  spectrum  of 
62  Angstrom  units  to  the  red  side  of  HS,  i.e.  from 
XX  4, 102-4, 164  are  given  in  the  following  table. 

"In  the  above  region,  where  ninety- two  lines  in  the 
flash  were  measured,  there  are  eighty-two  lines  in 
Rowland's  tables  of  an  intensity  2  and  greater.  Of 
these  eighty-two  lines,  but  one  is  with  certainty  lack- 
ing from  the  flash  spectrum,  the  Fe  line  (intensity  4) 
at  X  4,154.976.  Of  the  ninety-two  flash  lines,  all  have 
been  identified  with  the  exception  of  a  few  faint  lines. 
The  remarkable  accuracy  of  the  wave  lengths  of  this 
flash  spectrum,  which  far  surpasses  any  results  hitjh- 

177 


THE  SUN 


TABLE  X. — Measures  of  ninety-two  lines  in  the  flash  spectrum  near  HS. 


Flash 

Spectrum 

Inten- 

In- 
ten- 
sity 

Wave 
Length 

vVave 
Length 
Rowland 

Number 
of  Lines 
Blended 

Sub- 
stance 

sity 
and 
Char- 
acter 

Remarks 

5o 

4102.00 

4102.000 

H8 

40  N 

1 

4103.10 

4103.097 

Si,  Mn 

5 

0 

4103.65 

4103.622 

2 

1 

2 

4104.27 

4104.288 

Fe 

5 

0 

4104.65 

4104.623 

Co,  V, 

0 

3 

4105.21 

4105.245 

2 

—  ,  v 

3 

2 

4106.49 

4106.502 

2 

Fe 

4 

2 

4107.64 

4107.649 

Ce-Fe-Zr 

5 

0 

4108.68 

4108.687 

2 

3 

4109.37) 

4109.215) 

Fe 

3 

3 

4109.88) 

4  109.  609  j 

Nd? 

1 

4109.905 

V 

2 

2 

4li6!63 

4110.691 

Co 

4 

1 

4111.62 

2 

4111.97 

41  11.  '940 

V* 

4 

0 

4112.45 

4112.478 

Fe 

2 

0 

4112.89 

4112.869 

Ti 

1 

1 

4113.24 

4113.183 

2 

Fe,  Mn 

4 

2d 

4114  00 

3 

4114.73 

4114.769 

2 

Fe,'  — 

6  " 

3 

4115.35 

4115.330 

V 

3 

1 

4116.14 

4116.138 

0 

2d 

4116.78 

4116.738 

3' 

V.'Nd? 

2 

1 

4118.02 

4118.008 

2 

5 

4118.85 

4118.852 

3 

Fe.'Co 

11 

0 

4119.53 

4119.550) 

Fe 

1 

0 

4119.74 

4119.751V 

2 

... 

1 

0 

4120.12 

4120.075) 

0 

1 

4120.35 

4120.368 

Fe 

4 

2 

4120.93 

(4120.973) 

He 

Helium  line  at  4  120.  973 

3 

4121.46 

4121.477 

Cr-Co 

6d? 

Id 

4122.02 

4122.049 

2 

Fe,  Ti,  Cr 

4 

3 

4122.80 

4122.819 

1 

5 

4123.45 

4123.477 

2 

La,  'Mn 

3 

3 

4123.93 

4123.907 

Fe 

5 

2 

4124.96 

4124.938 

2 

1 

4125.93 

4125.900 

3 

Fe,'  — 

7 

1 

4126.35 

4126.344 

Fe 

4 

1 

4126.66 

4126.673 

Cr 

2 

5 

4127.86 

4127.872 

2 

Fe 

8 

5 

4128.25 

4128.251 

Ce-V 

6d? 

0 

4128.91 

4128.894 

2 

1 

4129.41 

4129.448 

2 

Ce'  — 

5 

5 

4129.88 

4129.882 

Eu 

1 

2 

4130.83 

4130.804 

Ba 

2 

0 

4131.46 

3d 

4132.16 

4io2-io6l 

4132.235| 

V 
Fe 

2V 

10  ; 

1 

4133.05 

4133.062 

Fe 

4 

2d 

4133.93 

4133.908 

3 

Fe,  Ce 

5 

2 

4134.49 

4134.492 

Fe 

3 

5 

4134.84 

4134.840 

Fe 

5 

2d? 

4135.56 

4135.529 

2 

1 

1 

4136.02 

2 

4136.69 

4i36!678 

Fe 

4 

3 

4137.26 

4137.156) 
4137.567) 

Fe 

6) 
2J 

4 

4137.79 

4137.809 

Fe.'Ce 

1 

0 

4138.31 

4138.324 

2 

1 

178 


ECLIPSES  AND  SOLAR  ENVELOPES 

TABLE  X.— Continued. 


Flash  Spectrum 

Wave 
Length 
Rowland 

Number 
of  Lines 
Blended 

Sub- 
stance 

Inten- 
sity 
and 
Char- 
acter 

Remarks 

In- 
ten- 
sity 

Wave 

Length 

1 

4139.08 

4139.008 

0 

0 

4139.57 

1 

4140.24 

4i40.245 

2 

Fe,'^ 

9 

1 

4141.81  1 

4141.809 

La 

01 

1 

4142.03 

4142.025 

Fe 

4 

. 

2 

4142.56  J 

4142.542 

4 

Cr,  — 

8j 

3 

4143.28 

(4143.30) 

Nd  line  at  4  143.  30 

6 

4144.05 

(4143.919)  1 

He 

Helium  line  at  4143.919 

4144.038  j 

Fe 

is 

2 

4144.63 

4144.674 

Ce 

ONd? 

2 

4145.13) 

4145.152 

Ce 

0) 

0 

4145.  37  J 

4145.357 

If 

4145.84 

4145.840 

2 

1 

3 

4146.23 

4146.225 

Fe 

3 

0 

4147.12 

4147.145 

2 

2 

4147.69 

4147.713 

3 

Mn,  Fe 

7 

4148.98 

4148.948 

Mn 

0 

10 

4149.37 

4149.360 

Zr 

2 

4149.923) 

2) 

[dentification  doubtful 

2 

4150.03 

4150.  056  J 

Ce 

oo; 

0 

4150.40 

4150.411 

4 

1 

4150.68 

3 

4151.18 

4i5i.'i29 

Zr,  Ti 

i 

6 

4152.23 

4152.248 

3 

La,  Fe,  Ce 

6 

0 

4152.68 

C 

0 

4153.51 

4i53.'542 

Fe 

i 

2d 

4154.09 

4154.112 

2 

Cr,  Fe 

5 

3 

4154.65 

4154.667 

Fe 

4 

6 

4156.30 

4156.339 

4 

Nd,  Zr 

5 

3 

4157.00 

4156.970 

Fe 

3d? 

3 

4158.00 

4157.948 

Fe 

5 

2d 

4159.00 

4158.959 

Fe 

5 

0 

4159.40 

4159.353 

5 

Od 

4160.57 

4160.53 

2 

lasselburg  gives  V 

4160.57 

1 

4161.231 

4161.239 

2 

5 

4161.  65  J 

4161.682 

Ti 

4 

Spark  line  Ti 

2 

4162.79 

4162.724 

2 

2N 

10 

4163.82 

4163.818 

Ti.'Cr 

Spark  line  Ti 

erto  published  makes  the  identification  of  lines  a 
practical  certainty.  Hence,  it  must  be  concluded 
that  the  flash  spectrum  is  a  reversal  of  the  Fraun- 
hofer  spectrum,  but  with  marked  differences  in  the 
intensities  in  the  two  spectra. 

"  Measures  of  the  1905  spectra  confirm  Mitchell's 
1901  results  that  hydrogen  (H),  helium  (He),  scan- 

179 


THE  SUN 

dium  (Sc),  titanium  (Ti),  strontium  (Sr),  vanadium 
(V),  Zr,  Y,  Cr,  Mn,  Nd  and  Ce  appear  with  a  greater 
intensity  in  the  flash  than  in  the  photospheric  spec- 
trum, relative  to  the  other  elements.  These  eclipse 
results  also  confirm  the  prominence  of  enhanced 
lines." 

Campbell's  Observations. 

Professor  Campbell  has  invented,  and  used  success- 
fully at  several  recent  eclipses,  a  spectroscope  with  a 
moving  plate.  He  begins  exposures  slightly  before 
totality  comes,  and  as  the  plate  keeps  falling,  the 
spectra  are  produced  in  a  continuous  series  at  deter- 
minable  times;  and  thus  are  adapted  to  give  the 
whole  history  of  the  spectrum,  as  it  changes  from  the 
photospheric  spectrum  (reflected  by  the  air)  to  the 
chromospheric,  or  flash  spectrum.  It  is  well  known 
that  Professor  Campbell  and  other  members  of  the 
Lick  expeditions  have  secured  spectra  of  very  high 
excellence  with  this  and  other  apparatus  in  recent 
eclipses,  of  which  the  discussion  is  not  yet  com- 
pleted. Professor  Campbell's  full  publication,  and 
that  of  Mitchell,  are  awaited  with  keen  interest. 

CHROMOSPHERIC  SPECTRA  IN  FULL  DAYLIGHT. 

Recently  Adams,  at  the  Mount  Wilson  Solar  Ob- 
servatory, has  obtained  many  photographs  of  the 
chromospheric  spectrum  in  full  sunlight.  This 
method  surpasses  in  accuracy  of  wave-length  meas- 
urements, and  may  eventually  rival  in  detail  the  best 

180 


ECLIPSES  AND  SOLAR   ENVELOPES 

eclipse  " flash  spectra."  Adams'  observations  were 
made  with  the  60-foot-focus  tower  telescope,  and  the 
30-foot-focus  plane-grating  spectroscope.  Success  de- 
pended on  securing  excellent  definition  of  the  sun's 
image,  so  that  the  spectroscope  slit  might  be  held 
exactly  to  the  edge  of  the  limb,  without  the  light  of 
the  photosphere  " boiling  over,"  so  as  to  blot  out  the 
bright-line  spectrum.  The  chromosphere  is  a  stratum 
so  thin  that  it  is  covered  by  the  march  of  the  moon  at 
an  eclipse  in  a  very  few  seconds.  Accordingly,  there 
is  not  sufficient  time  during  total  eclipses  for  the  ex- 
posure of  a  slit  spectrograph  of  high  dispersion,  and 
for  this  reason  slitless  spectrographs  of  moderate  dis- 
persion have  usually  been  employed.  Consequently, 
it  is  not  practicable  to  get  from  eclipse  "  flash  spectra" 
such  high  precision  of  wave  lengths  as  is  necessary  to 
decide  the  subtler  points  regarding  the  condition  and 
nature  of  the  chromosphere.  Hence,  the  great  advan- 
tage of  supplementing  eclipse  work  by  observations  at 
great  dispersion  in  full  sunlight,  especially  for  the  red 
end  of  the  spectrum,  where  photography  requires  long 
exposures.  Mr.  Hale  proposes  to  continue  the  work 
begun  by  Adams  with  the  60-foot  tower  telescope, 
and  is  making  provision  for  an  enlarged  solar  image. 
He  expects  greatly  enriched  results  when  the  150-foot 
tower  telescope  is  available. 

In  the  work  thus  far  published  by  Hale  and  Adams 
the  number  of  bright  lines  shown  is  far  less  than 
Mitchell  obtained  at  the  eclipse  of  1905.  Certain  dif- 
ferences seem  to  indicate  that  the  level  of  the  spectra 

181 


THE  SUN 

photographed  in  full  daylight  is  a  little  above  the 
level  best  observed  at  eclipses.  The  wave  length  of 
the  bright  lines  found  seem  to  be  practically  identical 
with  the  wave  lengths  of  corresponding  dark  lines  in 
the  photosphere.  This  circumstance,  as  Hale  and 
Adams' remark,  does  not  lend  support  to  Julius's  con- 
tention*, which  will  be  noted  in  a  later  chapter,  that 
the  bright  lines  of  the  " flash  spectrum"  are  due  to 
anomalous  refraction  of  light  just  outside  the.  sun's 
limb.  For,  if  this  were  the  case,  there  would  probably 
be  a  shifting  towards  the  red  of  their  apparent  wave 
lengths  from  those  of  the  dark  lines  of  the  photo- 
sphere. But  Julius  thinks  the  margins  of  discrepancy 
between  the  positions  of  the  bright  and  dark  lines,  as 
given  by  Hale  and  Adams,  still  leave  ground  for  his 
theory  of  anomalous  dispersion.  In  order  to  permit 
of  this  interpretation,  however,  Julius  imagines  "the 
solar  atmosphere  to  be  honeycombed  with  irregular 
density  gradients,  which  may  be  steeper  than  the 
underlying  general  radial  gradient."  Thus  he  finds 
the  possibility  that  displacements  of  the  chromo- 
spheric  lines  may  be,  now  to  the  red,  now  to  the  vio- 
let, of  the  Fraunhofer  lines.  Most  observers  still 
retain  the  view  that  the  chromospheric  spectrum  is 
essentially  the  reverse  of  the  Fraunhofer  spectrum 
and  appears  at  the  edge  of  the  sun  bright  instead  of 
dark  because  there  is  no  such  enormously  brilliant 
spectrum  background  to  dim,  by  comparison,  the  in- 
trinsic brightness  of  the  lines  themselves. 


182 


CHAPTER  V 

SUN-SPOTS,    FACUL^E,    AND    GRANULATION 

Sun-spot  Periodicity. — Drift. — Distribution  of  Sun-spots. — For- 
mation and  Life  History. — Sun-spot  Level. — Langley's  Typical 
Sun-spot . — Faculse. — Granulation. — Sun-spot  Spectra. — Cool- 
ness of  Sun-spots. — Sun-spots  and  Magnetism. — Radial  Motion 
in  Spot  Penumbras. 

ALTHOUGH  occasionally  seen,  and  recorded  much 
earlier  without  recognition  of  their  solar  origin,  the 
history  of  sun-spots  as  'solar  phenomena  dates  from 
1610,  when  they  were  independently  discovered  by 
Fabricius,  Scheiner,  and  Galileo.  The  discovery  fol- 
lowed naturally  from  the  invention  of  the  telescope 
in  Holland,  in  1608.  There  was  at  first  some  doubt 
(not  shared  by  Fabricius  or  Galileo)  whether  the  sun- 
spots  were  not  planets.  Indeed,  sun-spots  were  for  a 
time  called  in  France  the  "Bourbonian  Stars." 

Viewed  in  a  telescope,  or  projected  on  a  screen,  the 
sun-spots  are  plainly  seen,  and  appear  to  consist  of 
two  well-marked  parts;  the  umbra,  apparently  very 
dark,  and  the  penumbra,  a  half-tone  border  around 
the  umbra.  Sun-spots  differ  greatly  in  size,  shape, 
and  darkness.  Some  large  ones  are  2T  of  the  sun's 
diameter,  or  five  times  the  diameter  of  the  earth,  and 
sun-spot  groups  occasionally  spread  over  an  area  of 
H  183 


THE   SUN 

more  than  yV  the  sun's  diameter.     These  great  spots 
and  spotted  areas  are  rare. 

SUN-SPOT  PERIODICITY 

Schwabe  of  Desau  about  1843  discovered,  as  the 
result  of  systematic  observing  for  nearly  twenty 
years,  that  there  is  a  periodicity  in  the  occurrence  of 
sun-spots.  They  are  most  frequent  at  intervals  of 
about  eleven  years,  and  are  nearly  absent  for  a  year 
or  two  in  the  interim.  This  sun-spot  periodicity  was 
exhaustively  studied  by  Wolf  of  Zurich,  who  repre- 
sented the  spottedness  by  a  system  now  called 
"Wolfs  sun-spot  relative  numbers. "  These  are  com- 
puted by  the  formula,  r  =  k(Wg  +  /),  in  which  r  is 
Wolf's  number,  g  the  number  of  groups  and  single 
spots  observed,  /  the  total  number  of  spots  which  can 
be  counted  in  these  groups  and  single  spots  combined, 
and  k  a  multiplier  which  depends  on  the  conditions 
of  observation  and  the  telescope  employed.  Wolf  took 
k  as  unity  for  himself  when  observing  with  a  three- 
inch  telescope  and  a  power  of  64.  A  less  favored  or 
less  assiduous  observer  would  receive  k  greater 
than  unity,  and  one  with  a  larger  telescope  and  good 
opportunities  for  observing  would  receive  a  fractional 
value  of  k.  Wolf's  numbers  seem  arbitrary,  but  are 
found  by  photographic  comparisons  to  be  closely  pro- 
portional to  the  spotted  areas  on  the  sun.  One  hun- 
dred as  a  sun-spot  number  corresponds  to  about  5-^0 
of  the  sun's  visible  disk  covered  by  spots,  including 
both  umbras  and  penumbras. 

184: 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 

Wolf,  by  consulting  all  available  sources,  carried 
his  sun-spot  numbers  back  to  1610.  His  successor, 
Wolfer,  has  kept  up  the  series  from  Wolf's  death,  in 
1893,  up  to  the  present  time.  In  Fig.  53  and  Fig.  54 
the  curves  show  the  run  of  spottedness  during  all  this 
interval.1  It  will  be  seen  that  the  maxima  and  min- 
ima are  not  uniformly  spaced;  but  so  that,  while  a 
mean  sun-spot  interval  of  11.13  years  is  deduced  by 
Professor  Newcomb,  the  individual  periods  range 
between  7.3  and  17.1  years  as  extremes.  These  fea- 
tures are  shown  in  the  table  on  page  186. 

Newcomb  finds  the  average  time  of  increasing 
spottedness,  4.62  years,  of  decreasing  spottedness, 
6.51  years.  Having  studied  the  total  interval  from 
1610  to  1898  in  three  parts,  he  concludes  that:  " Un- 
derlying the  periodic  variations  of  spot  activity 
there  is  a  uniform  cycle,  unchanging  from  time 
to  time,  and  determining  the  general  mean  of  the 
activity." 

The  reader  will  note  in  the  sun-spot  curves  that 
there  is  not  only  a  great  dissimilarity  in  the  lengths  of 
the  individual  periods,  but  also  of  their  activities  as 
measured  by  the  maximum  number  of  spots  observed. 
Dr.  Lockyer  pointed  out  a  relation  between  these 
phenomena  which  has  been  mentioned  also  by  Halm 
and  thoroughly  confirmed  by  Wolfer.  Call  the  time 

1  In  Fig.  53  the  inclusion  of  new  data  leads  to  a  modification  as 
follows: 

Year.... 1800     1801     1802     1803     1804     1805     1806 
Mean... 15.0    33.7     44.1     43.0     46.8     42.5     27.3 
These  data  are  corresponding  to  the  mid-years. 

185 


THE  SUN 


TABLE  XI. — Years  of  sun-spot  maxima  and  minima  and  maximum 

intensities 


Minima 

Difference 

Maxima 

Difference 

Maximum 
Wolf  Number 

1610.8 

1615.5 

1619.0 

Y.2 

1626.0 

10'.5 

1634.0 

15.0 

1639.5 

13.5 

1645.0 

11.0 

1649.0 

9.5 

1655.0 

10.0 

1660.0 

11.0 

1666.0 

11.0 

1675.0 

15.0 

1679.5 

13.5 

1685.0 

10.0 

1689.5 

10.0 

1693.0 

8.0 

1698.0 

8.5 

1705.5 

12.5 

1712.0    . 

14.0 

1718.2 

12.7 

1723.5 

11.5 

1727.5 

9.3 

1734.0 

10.5 

1738.7 

11.2 

1745.0 

11.0 

1750.3 

11.6 

'83 

1755.2 

11.2 

1761.5 

11.2 

80 

1766.5 

11.3 

1769.7 

8.2 

103 

1775.5 

9.0 

1778.4 

8.7 

151 

1784.7 

9.2 

1788.1 

9.7 

133 

1798.3 

13.6 

1805.2 

17.1 

47 

1810.6 

12.3 

1816.4 

11.2 

46 

1823.3 

12.7 

1829.9 

13.5 

67 

1833.9 

10.6 

1837.2 

7.3 

137 

1843.5 

9.6 

1848.1 

10.9 

125 

1856.0 

12.5 

1860.1 

12.0 

95 

1867.2 

11.2 

1870.6 

10.5 

132 

1878.9 

11.7 

1883.9 

13.3 

65 

1889.6 

10.7 

1894.1 

10.2 

84 

1901.6 

12.0 

1906.4 

12.3 

60 

interval  from  a.minimum  to  the  following  maximum  a, 
and  that  from  the  maximum  to  the  following  mini- 
mum b.  The  variations  of  a  and  of  the  ratio  -  proceed 

0 

in  a  sense  contrary  to  the  intensity  of  the  outbreak  of 
spottedness  in  each  period.  In  other  words,  the  more 
intense  the  outbreak  of  spots  in  any  sun-spot  period 
the  shorter  the  time  required  for  its  development, 

186 


SUN-SPOTS,   FACUL.E,   AND   GRANULATION 


187 


THE   SUN 

both  actually  and  as  compared  with  the  time  re- 
quired for  its  decay. 

It  is  interesting  to  observe,  also,  that  the  interval 
from  minimum  to  maximum  is  always  much  less  than 
that  from  maximum  to  minimum.  Attention  will  be 
drawn  in  Chapter  X  to  the  similarity  between  this 
characteristic  and  a  certain  type  of  stellar  variation 
exemplified  in  the  star  Mira. 

The  causes  which  produce  sun-spots  being  as  yet 
doubtful,  or  perhaps  it  is  better  to  say  entirely  un- 
known, the  causes  of  their  periodicity  and  of  the  ir- 
regularity of  the  periods  are,  of  course,  also  unknown. 
Attempts  have  been  made  to  connect  the  period  with 
the  times  of  revolution  of  the  planets,  and,  indeed, 
the  mean  length  of  the  sun-spot  period  is  not  far  from 
the  period  of  the  revolution  of  Jupiter  (11.86  years). 
No  satisfactory  case  for  a  connection  between  these 
phenomena  is  yet  made  out.  Schuster  has  recently 
applied  a  method  of  mathematical  analysis  fitted  to 
bring  out  secondary  periods  which  may  underlie  the 
average  sun-spot  periodicity  of  1 1 . 1 3  years.  He  finds 
three  well-marked  periods  of  11.125  years,  8.32  years, 
and  4.77  years.  As  curious  mathematical  coinci- 
dences he  notes  that  the  sum  of  the  reciprocals  of  the 
first  two  periods  equals  the  reciprocal  of  the  third, 
and  all  three  are  nearly  even  fractions  of  33f 
years.  He  finds  the  relative  intensities  of  spot- 
tedness  for  his  three  periods  variable,  hence  the 
inequality  of  the  successive  total  periods  produced 
by  their  combination.  He  inclines  to  attribute 

188 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 

sun-spots    to    causes    outside   the  sun,   perhaps  to 
meteor  swarms. 

Several  kinds  of  phenomena,  some  solar,  others  ter- 
restrial, are  evidently  closely  associated  with  sun- 
spots  and  share  in  their  periodicity.  Firstly,  the  f  acu- 
lae,  or  bright  flecks  on  the  solar  surface,  which  are 
always  seen  most  plentifully  in  sun-spot  neighbor- 
hoods, naturally  have  the  same  seasons  of  maxima 
and  minima.  Secondly,  the  prominences,  as  stated 
in  the  preceding  chapter,  are  most  numerous  at  sun- 
spot  maxima,  and  decrease  in  number,  though  not 
with  so  marked  a  change,  as  the  number  of  sun-spots 
decreases.  Thirdly,  the  form  of  the  solar  corona  evi- 
dently goes  through  a  periodic  change  simultaneous 
with  the  sun-spot  cycle.  Thus  we  speak  of  a  solar 
corona  having  prolonged  equatorial  streamers  of  an 
arrow-head  shape  as  "a  sun-spot  minimum  corona," 
and  one  nearly  equally  developed  in  all  directions  as 
"a  sun-spot  maximum  corona."  Fourthly,  the  ter- 
restrial auroras  (northern  and  southern  lights)  follow 
the  sun-spot  periodicity,  as  shown  by  Loomis  and 
many  others.  Fifthly,  changes  in  the  earth's  magnetic 
field  occur  in  complete  synchronism  with  the  changes 
of  sun-spot  numbers.  This  connection  is  very  close, 
for  the  agreement  descends  even  to  minute  parallel- 
ism, as  shown  by  the  magnetic  curves  plotted  in  Fig. 
53  and  Fig.  54.  Great  sun-spots  often  seem  to  be  the 
direct  promoters  of  great  magnetic  disturbances 
(magnetic  storms)  and  auroral  displays.  Maunder 
has  found  that  the  magnetic  disturbances  seem  to 

189 


THE  SUN 

arise  from  restricted  solar  areas,  not  necessarily 
including  sun-spots,  and  fo  go  out  in  definite 
directions,  or  rather  shafts  of  several  degrees 
diameter,  which  rotate  with  the  sun.1  When  such 
a  shaft  strikes  the  earth  a  magnetic  storm  arises. 
Such  lines  of  influence  are  not,  he  thinks,  neces- 
sarily radial,  but  may  follow  coronal  stream  lines. 
Sixthly,  the  earth's  surface  air  temperature  is  on  the 
whole  lower  at  sun-spot  maximum  than  at  sun-spot 
minimum.  This  relation  is  indicated  at  least  for 
the  United  States  in  Fig.  54.  The  difference  of  mean 
temperature  for  the  earth  generally,  ranging  from  0°.5 
to  1°.0  Centigrade  for  a  change  of  100-sun-spot  num- 
bers, is  shown  by  temperature  statistics  studied  by 
Koppen,  Nordmann,  Newcomb,  Abbot,  Fowle,  Arc- 
towski,  and  Bigelow.  This  will  be  further  discussed  in 
Chapter  VII.  Many  other  terrestrial  changes,  in 
rainfall,  cloudiness,  number  of  cyclones,  panics,  prices 
of  foods,  famines,  growth  of  trees,  even  flights  of  in- 
sects, have  been  seriously  compared  with  the  sun- 
spot  changes.  In  some  of  these  phenomena  there 
appear  to  be  rather  well-substantiated  indications  of 
a  periodicity  coincident  with  that  of  sun-spots,  while 
such  relations  in  many  cases  are  probably  purely  fan- 
ciful. 

So-called  " great  periods"  of  33,  35,  55,  and  several 
hundred  years  have  been  proposed  by  various  au- 
thors in  order  to  explain  the  variations  of  the  lengths 

1  Shearman  of  Toronto  discovered  also  a  periodicity  of  auroral 
displays  approximating  the  rotation  period  of  the  sun. 

190 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 


/  * 

9 

1 

, 

/ 

f 

,  —  • 

-y 

^ 

2 

2 

5 

V 

—  «. 

^ 

/ 

{ 

j 

< 

^ 

^ 

/ 

7 

^ 

/ 

r 

r- 

^ 

. 

^ 

<5 

y 

1 

Jr 

r 

f 

• 

^ 

/ 

/ 

2 

/ 

I*** 

• 

( 

( 

\ 

c; 

i 

V 

\ 

( 

\ 

X 

X 

\ 

^ 

^ 

i 

^  -j 

5 

\ 

5 

k<c. 

3T 

I 

i 

^ 

1 

/ 

^« 

*^ 

/ 

jf 

'« 

/ 

/ 

^ 

/ 

<: 

./• 

7 

. 

i 

i 

> 
^> 

s 

^ 

r 

/^ 

^ 

s 

\ 

* 

i 

< 

*v- 
\ 

—  ^»_ 

' 

/ 

[ 

xx 

"X^, 

\ 

\ 

s 

v 

*>  — 

ss 

~~\ 

V 

\ 

X 

V 

\ 

*x. 

.^    '•* 

r> 

\ 

^ 

\ 

^  — 

r 

^- 

\ 
' 

1 

-j- 

-f—* 

7 

S-31 


i  an 

lias, 

c  n  eg  os 


P    .<O    .O.     ,«1    O     o     O     O     O     O      Q     Q     O     —     00     in 

f  Y Jo   ?  + 1"0   ^   ^      »2        °°  ^  I  g  5  5 


191 


THE   SUN 

and  intensities  of  the  eleven  year  periods,  and  the 
changes  of  rainfall,  times  of  harvest,  and  other 
changes  of  terrestrial  phenomena  said  to  be  indicated 
by  history  or  tradition.  The  tendency  to  groups  of 
three  in  respect  to  the  intensity  of  the  successive  sun- 
spot  outbreaks  has  been  mentioned  by  various  writ- 
ers, and  may  be  noted  in  the  table.  This  helps  to  sus- 
tain belief  in  a  thirty-three  year  period,  but  it  will  be 
noted  that  the  four  maxima  1830  to  1870  were  un- 
commonly intense  (perhaps  excepting  the  intermedi- 
ate one  of  1860) .  The  question  of  the  reality  of  "  great 
periods"  seem  to  require  further  lapse  of  years  to  de- 
cide it. 

SUN-SPOT  DRIFT 

If  we  imagine  an  observer  on  the  moon  to  watch  the 
clouds  on  the  earth's  surface,  they  would  appear  to 
him  on  the  whole  to  indicate  a  mean  rotation  period 
of  about  twenty-four  hours  for  the  earth.  But  he 
would  also  discover  that  many,  and  perhaps  nearly 
all,  of  the  cloudy  areas  had  proper  motions  of  their 
own  besides,  so  that  no  single  cloud  would  give  cor- 
rectly the  rotation  period  of  the  earth.  So  it  is  with 
the  sun-spots,  for,  after  allowing  for  the  sun's  average 
rotation  period,  nearly  every  spot  has  a  motion  of  its 
own.  Carrington  found  a  slight  tendency  of  spots  be- 
tween 20°  North  and  20°  South  latitudes  to  approach 
the  equator,  and  outside  these  latitudes  a  more  de- 
cided tendency  to  approach  the  poles.  Faye  held 
that  spots  persistently  describe  little  ellipses  on  the 
sun's  surface  of  one  or  two  days'  period.  It  is  said 

192 


SUN-SPOTS,  FACUL.E,   AND   GRANULATION 

that  an  actively  changing  spot  is  apt  to  move  forward 
by  irregular  jerks.  When  a  spot  divides,  the  parts  are 
apt  to  separate  rapidly. 

DISTRIBUTION  OF  SUN-SPOTS 
Sun-spots  very  seldom  occur  at  higher  latitudes 
than  40°.  Within  the  sun-spot  belt  80°  wide,  as  thus 
defined,  the  distribution  of  spots  is  irregular.  They 
occur  mainly  in  two  zones  on  either  side  of  the  equa- 
tor, between  latitudes  10°  and  30°.  As  regards  the 
northern  and  southern  hemispheres  the  number  oc- 
curring in  a  very  long  period  of  years  is  practically 
equal,  but  there  is  often  a  great  inequality  for  several 
years  in  succession.  A  remarkable  instance  of  this 
irregularity  occurred  between  1672  and  1704,  when 
no  spots  were  recorded  in  the  northern  hemisphere, 
and  the  appearance  of  a  few  there  in  1705  was  noted 
by  the  French  Academy  as  a  very  extraordinary 
event.  Newcomb  draws  attention  in  the  four  cycles 
1856-1898  to  a  marked  and  growing  preponderance 
of  spots  in  the  southern  hemisphere.  A  peculiarity 
of  sun-spot  distribution  likely  to  prove  of  great  theo- 
retical significance  was  discovered  by  Spoerer,  and  is 
confirmed  by  Greenwich  observations.  There  seems 
to  be  a  close  connection  between  the  latitudes  of 
great  prevalence  and  the  periodicity  of  sun-spots. 
Young  states  the  matter  as  follows : 

"  Speaking  broadly,  the  disturbance  which  pro- 
duces the  spots  of  a  given  sun-spot  period  first  mani- 
fests itself  in  two  belts  about  30°  north  and  south  of 

193 


THE  SUN 


the  sun's  equator.  These  belts  then  draw  in  toward 
the  equator,  and  the  sun-spot  maximum  occurs  when 
their  latitude  is  about  16°;  while  the  disturbance 
gradually  and  finally  dies  out  at  a  latitude  of  8°  or  10°, 
some  twelve  or  fourteen  years  after  its  first  outbreak. 
Two  or  three  years  before  this  disappearance,  how- 
ever, two  new  zones  of  disturbance  show  themselves. 
Thus,  at  the  sun-spot  minimum  there  are  four  well- 
marked  spot-belts;  two  near  the  equator,  due  to  the 
expiring  disturbance,  and  two  in  high  latitudes,  due 


18 

55                  18 

60                 18 

65                  18 

70                   18 

75                  18 

80 

L. 

30I 
26 

22° 
18° 

14° 
10° 

/ 

f\ 

\ 

W. 
100 

50 
0 

\ 

\ 

< 

/ 

N 

\ 

s 

X 

7 
s 

\ 

-- 

--'• 

/' 

X 

X 

^ 

^«. 

•^ 

s»^ 

"X 

\ 

^ 

^ 

^ 

\. 

S 

^ 

/ 

FIG.  55. — SPOERER'S  CURVES  OF  SUN-SPOT  LATITUDE. 

to  the  newly  beginning  outbreak;  and  it  appears  that 
the  true  sun-spot  cycle  is  from  twelve  to  fourteen 
years  long,  each  beginning  in  high  latitudes  before 
the  preceding  one  has  expired  near  the  equator. 

"  Fig.  55  illustrates  this,  embodying  Spoerer's  results 
from  1855  to  1880.  The  dotted  curves  show  Wolf's 
sun-spot  curve  for  that  period,  the  vertical  column  at 
the  right  of  the  figure,  marked  W  at  the  top,  giving 
Wolf's  '  relative  numbers. '  The  two  continuous  curves, 
on  the  other  hand,  give  the  solar  latitudes  of  the  two 
series  of  spots  that  invaded  the  sun's  surface  in  those 
years.  The  scale  of  latitudes  is  on  the  left  hand.  The 

194 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 


first  series  began  in  1856  and  ended  in  1868;  the  sec- 
ond broke  out  in  1866  and  lasted  until  1880.  During 
these  years  it  happened  that  there  was  very  little  dif- 
ference between  the  northern  and  southern  hemi- 
spheres of  the  sun. " 

In  a  summary  of  the  results  of  solar  observations 
made  at  Greenwich  from  1874  to  1902  the  Astrono- 
mer Royal,  Christie,  gives  data  showing  the  prevail- 
ing latitudes  at  which  they  occurred  in  different  parts 
of  the  sun-spot  cycles.  The  maximum  latitude  at 
which  spots  occurred  was  42°,  but  they  could  only  be 
regarded  as  sporadic  phenomena  above  latitude  33°. 
Preceding  a  time  of  sun-spot  minimum,  the  prevailing 
spottedness  occurred  at  low  latitudes,  and  when  the 
spots  reappeared  after  minimum  it  was  generally  at 
high  latitudes.  The  equatorial  belt  from  5°  to  —5° 
was  never  a  center  of  spottedness.  These  facts  are 
indicated  by  the  following  table,  abridged  from  the 
data  given  by  the  Astronomer  Royal. 


Year 

1S^ 

80 

8? 

84 

86 

91 

Q3 

9") 

97 

Centers  of  

N 

21° 

16° 

11° 

9° 

21° 

15° 

12° 

8° 

spottedness  

S 

19° 

18° 

11° 

10° 

30° 

15° 

1?,° 

7° 

Wolf's  numbers  

32 

58 

63 

25 

38 

84 

62 

28 

The  protuberances,  on  the  contrary,  as  shown  by 
Ricco  and  the  Lockyers,  and  confirmed  by  Mascari, 
have  their  zones  of  maximum  frequency  transferred 
from  low  toward  higher  latitudes  as  the  sun-spot 
cycles  progress. 

195 


THE  SUN 

SUN-SPOT  FORMATION  AND  LIFE-HISTORY 

As  regards  the  formation  and  life  history  of  sun- 
spots,  Young  has  described  the  phenomena  in  these 
words  : 

"  There  is  no  regular  process  for  the  formation  of  a 
spot.  Sometimes  it  is  gradual,  requiring  days  or  even 
weeks  for  its  full  development,  and  sometimes  a  single 
day  suffices.  Generally,  for  some  time  before  the  ap- 
pearance of  the  spot,  there  is  an  evident  disturbance 
of  the  solar  surface,  manifested  especially  by  the  pres- 
ence of  numerous  and  brilliant  faculse,1  among  which, 
"pores"  or  minute  black  dots  are  scattered.  These 
enlarge,  and  between  them  appear  grayish,  patches, 
apparently  caused  by  a  dark  mass  lying  veiled  below 
a  thin  layer  of  luminous  filaments.  The  veil  grows 
gradually  thinner,  and  vanishes,  giving  us  at  last  the 
completed  spot  with  its  perfect  penumbra.  The 
"pores,"  some  of  them,  coalesce  with  the  principal 
spot,  some  disappear,  and  others  constitute  the  at- 
tendant train.  When  the  spot  is  once  completely 
formed,  it  assumes  usually  an  approximately  circular 
form,  and  remains  without  striking  change  until  its 
dissolution.  As  its  end  approaches,  the  surrounding 
photosphere  seems  to  crowd  in  upon  and  cover  and 
overwhelm  the  penumbra.  Bridges  of  light,  often 
many  times  brighter  than  the  average  of  the  solar 
surface,  push  across  the  umbra,  the  arrangement  of 


is  Secchi's  view.     Lockyer  maintains  that  the  spots  appear 
before  the  faculse. 

196 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 

the  penumbra  filaments  becomes  confused,  and,  as 
Secchi  expresses  it,  the  luminous  matter  of  the  photo- 
sphere seems  to  tumble  pell-mell  into  the  chasm, 
which  disappears  and  leaves  a  disturbed  surface 
marked  with  faculse,  which  in  their  turn  subside  after 
a  time.  As  intimated  before,  however,  the  disturb- 
ance is  not  unfrequently  renewed  at  the  same  point 
after  a  few  days,  and  a  fresh  spot  appears  just  where 
the  old  one  was  overwhelmed. 

"The  spots  usually  appear  not  singly,  but  in  groups 
—at  least,  isolated  spots  of  any  size  are  less  common 
than  groups.  Very  often  a  large  spot  is  followed  upon 
the  eastern  side  by  a  train  of  smaller  ones;  many  of 
which,  in  such  a  case,  are  apt  to  be  very  imperfect  in 
structure,  sometimes  showing  no  umbra  at  all,  often 
having  a  penumbra  only  upon  one  side,  and  usually 
irregular  in  form.  It  is  noticeable,  also,  that  in  such 
cases,  when  any  considerable  change  of  form  or  struc- 
ture shows  itself  in  the  principal  spot  of  a  group,  it 
seems  to  rush  forward  (westward)  upon  the  solar 
surface,  leaving  its  attendants  trailing  behind.  When 
a  large  spot  divides  into  two  or  more,  as  often  hap- 
pens, the  parts  usually  seem  to  repel  each  other  and 
fly  asunder  with  great  velocity — great,  that  is,  if 
reckoned  in  miles  per  hour,  though,  of  course,  to  a 
telescopic  observer  the  motion  is  very  slow,  since  one 
can  only  barely  see  upon  the  sun's  surface  a  change  of 
place  amounting  to  two  hundred  miles,  even  with  a 
very  high  magnifying  power.  Velocities  of  three  or 
four  hundred  miles  an  hour  are  usual,  and  velocities 

197 


THE  SUN 

of  one  thousand  miles,  and  even  more,  are  by  no 
means  exceptional. 

"The  average  life  of.  a  sun-spot  may  be  taken  as 
two  or  three  months;  the  longest  yet  on  record  is 
that  of  a  spot  observed  in  1840  and  1841,  which  lasted 
eighteen  months.  There  are  cases,  however,  where  the 
disappearance  of  a  spot  is  very  soon  followed  by  the 
appearance  of  another  at  the  same  point,  and  some- 
times this  alternate  disappearance  and  reappearance 
is  several  times  repeated.  While  some  spots  are  thus 
long-lived,  others,  however,  endure  only  for  a  day  or 
two,  and  sometimes  only  for  a  few  hours." 

Carrington,  Secchi,  Perry,  Maunder,  and  Sid- 
greaves  have  all  noted  the  tendency  of  spots  to  recur 
in  the  same  positions,  but  not  in  a  sense  indicative  of 
permanent  special  eruptive  places,  as  in  the  case  of 
terrestrial  volcanoes.  Father  Sidgreaves  says:  "They 
are  indications  of  a  more  enduring  state  of  disturb- 
ance than  is  measured  by  the  lifetime  of  a  single  spot, 
for  it  is  not  improbable  that  a  recurrence  springs  from 
the  same  source  as  its  predecessor.  And,  if  this  be 
true,  the  spots  must  be  more  subject  to  drift  than 
their  underlying  origins,  for  nearly  always  the  recur- 
ring spot  is  found  to  the  rear  of  the  former  position. " 

According  to  the  spectroheliographic  investiga- 
tions of  Fox:1  "Spot  birth  is  always  accompanied  by, 
and  generally  antedated  by,  an  eruption"  (i.  e.,  erup- 
tive prominence).  "In  the  early  hours  of  the  life  of  a 
spot  the  eruption  may  partially  or  entirely  cover  the 

lAstrophysical  Journal,  vol.  xxviii,  p.  255,  1908. 
198 


SUN-SPOTS,   FACUL.E,  AND  GRANULATION 

spot,  and  often  may  precede  it,  in  the  direction  of 
solar  rotation.  An  eruption  is  seldom  seen  preceding 
a  mature  single  spot,  but  if  present  will  be  following  it 
at  the  edge  of  the  penumbra,  perhaps  encroaching 
somewhat.  If  the  spot  is  actively  growing,  eruptions 
are  almost  certain  to  be  found  on  the  following  edge. 
Eruptions  accompany  spots  in  rapid  decline,  being 
often  seen  at  the  ends  of  the  bridges.  I  think  the 
evidence  of  the  Rumford  spectroheliograms  fairly 
conclusive  in  showing  that  the  spot  has  its  genesis  in 
the  eruption.  The  phenomenon  of  spot  development 
following  the  appearance  of  an  eruption  is  so  general 
that  it  is  possible,  on  the  appearance  of  an  isolated 
eruption,  to  predict  with  certainty  the  advent  of  a 
spot.  When  the  spot  is  well  developed  it  stimulates 
new  eruptions. "  The  "eruptions"  mentioned  by  Fox 
are,  of  course,  seen  with  the  spectroheliograph  any- 
where on  the  sun's  disk,  but  when  close  to  the  limb 
they  are  recognized  by  him  to  be  really  "the  bases  of 
the  eruptive  prominences. " 

THE  SUN-SPOT  LEVEL 

The  level  of  sun-spots  is  a  question  which  has  been 
discussed  for  over  a  century,  and  often  with  consider- 
able vehemence.  In  1769,  Dr.  A.  Wilson  of  Glas- 
gow advocated  the  view  that  sun-spots  are  depres- 
sions of  the  sun's  surface.  He  observed  that  when  a 
spot  first  appears  on  the  eastern  edge  of  the  sun  the 
penumbra  is  well  marked  on  the  side  nearest  the  edge 
of  the  sun,  but  nearly  invisible  on  the  side  next  the 
15  199 


THE  SUN 

sun's  center,  while  the  umbra  scarcely  shows  at  all, 
being  as  if  hidden  behind  a  bank.  As  the  spot  ad- 
vances towards  the  center,  according  to  A.  Wilson, 
the  advancing  and  following  sides  of  the  penumbra 
become  more  equal,  and  the  umbra  covers  an  increas- 
ing fraction  of  the  total  width  of  the  spot.  Having 
passed  the  center,  the  spot  naturally  exhibits  the  op- 
posite succession  of  phenomena.  This  progress  in  ap- 
pearance would  be  conclusive  evidence  that  spots  are 
depressions  if  it  were  universally  admitted  to  be  real. 
Many  spots  are  so  unsymmetrical,  even  at  the  center 
of  the  sun,  as  to  be  unfavorable  objects  on  which  to 
test  A.  Wilson's  view.  Manj^  spots  alter  their  shape  in 
crossing  the  sun's  disk  quite  apart  from  any  change 
due  to  the  sun's  spherical  form.  In  the  last  twenty 
years  several  very  assiduous  observers  have  published 
conclusions  based  on  very  numerous  observations; 
and,  even  when  discussing  the  spots  occurring  in  the 
same  course  of  years,  about  as  many  disagree  with  A. 
Wilson's  view  as  support  him.  It  seems  most  prob- 
able, therefore,  that  the  level  of  the  sun-spot  phe- 
nomena seen  by  ordinary  observation  differs  very 
little,  if  at  all,  from  that  of  the  surrounding  bright 
surface  of  the  sun. 

LANGLEY'S  TYPICAL  SUN-SPOT 

Owing  to  the  effect  of  the  sun's  rays  in  heating  the 
surface  of  the  earth,  and  thereby  causing  the  ascent  of 
warm  currents  of  air  which  spoil  the  " seeing,"  the 
observer  is  at  a  disadvantage  in  studying  the  minute 

200 


SUN-SPOTS,   FACUL^,  AND  GRANULATION 

features  of  the  sun  as  compared  with  the  moon  or 
other  night  objects.  The  "seeing"  on  the  sun  is  gen- 
erally better  in  the  hours  soon  after  sunrise  and  before 
sunset,  when  the  heating  of  the  sun's  rays  is  dimin- 
ished both  by  passing  through  a  thick  stratum  of  air 
and  by  striking  the  earth's  surface  obliquely.  Some- 
times the  presence  of  thick  haze  or  light  uniform 
cloudiness  appears  to  favor  good  definition,  but  often 
these  conditions  are  connected  with  atmospheric  dis- 
turbances so  nearly  in  line  with  the  sun  as  to  spoil  the 
"seeing. "  Good  solar  "seeing "  is  seldom  found  when 
a  clear  blue  sky,  a  brisk  breeze,  and  high  altitude  of 
the  sun  occur  simultaneously.  With  the  hindrance 
thus  occasioned  by  the  irregularities  of  density  in  the 
earth's  atmosphere  to  contend  with,  solar  observers, 
as  a  rule,  find  only  comparatively  rare  instants  when 
really  satisfactory  views  of  the  sun's  surface  may  be 
obtained.  By  combining  with  extraordinary  skill 
the  impressions  received  in  the  instants  of '  best 
"seeing,"  which  were  the  reward  of  several  years  of 
assiduous  observing,  the  late  Dr.  S.  P.  Langley 
produced,  in  1873,  his  famous  sketch  of  the  "typical 
sun-spot,"  a  copy  of  which  is  reproduced  as  the 
frontispiece.  This  is  generally  conceded  to  represent 
better  than  any  photographs,  and  even  better  than 
anyone  is  likely  to  see  for  himself  in  the  telescope, 
the  appearance  of  a  sun-spot  and  its  surroundings  as 
seen  under  the  best  purely  telescopic  observation. 


201 


THE  SUN 

FACULSE 

Next  to  sun-spots,  the  most  prominent  solar  fea- 
tures, and  closely  associated  with  the  life  history  of 
spots,  are  the  faculse,  or  bright  patches  which  are 
most  abundantly  seen  near  the  borders  of  the  sun's 
disk.  Their  appearance  has  been  likened  by  Young 
to  the  flecks  of  foam  which  dot  the  water  beneath  a 
waterfall.  They  are  very  prevalent  in  the  neigh- 
borhood of  sun-spots,  but,  unlike  them,  they  are 
found  all  over  the  surface  of  the  sun,  though  spar- 
ingly near  the  poles.  It  is  difficult  to  see  them  near 
the  center  of  the  sun's  disk.  As  stated  in  Chapter 
III,  the  brilliancy  of  the  solar  surface  is  not  uniform 
all  over  the  disk,  but  falls  off  very  greatly  near  the 
edges.  Speaking  roughly,  the  faculse,  on  the  other 
hand,  may  be  regarded  as  equally  bright  wherever 
seen  on  the  sun's  disk,  and  hence  come  out  more 
distinctly  near  the  edges,  where  the  background  is 
less  brilliant.  The  prevalence  of  faculse  has  maxima 
and  minima  synchronous  with  the  sun-spot  period. 

GRANULATION 

Besides  the  sun-spots  and  the  faculse,  there  is  seen 
under  good  observing  conditions  a  general  granu- 
lated appearance  all  over  the  sun's  surface.  Many 
years  ago  much  controversy  was  waged  over  the 
exact  forms  of  the  granules,  some  observers  compar- 
ing them  to  rice  grains,  others  to  willow  leaves,  and 
others  to  bits  of  straw.  These  patches  of  differing 

202 


PLATE  XVI 


6  h  47  ra. 


7  h  37m. 

PHOTOGRAPHS  OF  A  PORTION  OF»  THE  SUN.    (Janssen.) 
Meudon,  June  1,  1878.     Interval,  50  minutes. 


SUN-SPOTS,   FACUL.E,   AND   GRANULATION 


brilliance  are  really  immense  areas  of  10,000  to  50,000 
square  miles,  and  are  probably  not  of  a  regular  pat- 
tern at  all,  so  that  little  insight  into  solar  conditions 
is  had  by  the  discussion  of  their  mere  forms.  In 
Langley's  sun-spot  drawing  they  are  depicted  in  great 
numbers,  and  with  various  shapes,  quite  as  they  are 
apt  to  occur.  Plate  XVI  is  a  reproduction  of  two  of 
Janssen's  celebrated  photographs  of  them. 

SUN-SPOT  SPECTRA 

The  spectrum  of  a  sun-spot  differs  from  that  of  the 
photosphere  in  several  significant  ways.  (1)  As  meas- 
ured by  the  bolometer  or  other  photometric  methods, 
its  energy  is  far  weaker  in  the  violet.  This  is  shown  in 
the  accompanying  comparisons  between  the  intensi- 
ties of  the  spectra  of  sun-spots  and  of  the  photosphere 
near  the  center  of  the  sun's  disk.  The  data  for  the 
ultra-violet  spectrum  are  from  the  work  of  Schwartz- 
child  and  Villiger,  and  the  remainder  from  the  work 
of  the  Smithsonian  observers. 


Wave  Lengths  (A.  =  ) 

Oju.320 

O/x.448 

O/i.586 

Ojt.799 

l/i.218 

2/*.115 

Ratio  of           umbra 

0.12 

0.377 

0.424 

0.535 

0.610 

0.761 

brightness   photosphere 

As  different  spots  differ  in  darkness  of  their  centers, 
too  much  reliance  should  not  be  placed  on  the  transi- 
tion of  relative  brightness  from  X  =  0.320/z  to  X  = 
0.448/x,  as  given  above.  The  remainder  of  the  data, 
however,  all  applies  to  the  same  spot  observed  by  the 

203 


THE  SUN 

same  observers,  and  should,  therefore,  be  comparable. 
There  are  three  ways  of  explaining  the  progressive 
relative  weakness  of  the  shorter  wave-length  rays  in 
sun-spots.  The  sun-spot  temperatures  may  be  much 
below  those  of  the  photosphere,  there  may  be  a 
greater  amount  of  absorption  or  scattering  of  the 
light  above  the  spots,  or,  finally,  the  phenomenon 
may  be  due  to  the  action  of  both  these  causes.  It  has 
lately  been  made  practically  certain  that  the  first- 
mentioned  cause,  at  least,  is  operative.  This  is 
proved  by  the  work  on  sun-spot  spectra  noted  below. 
Several  observers  have  found  that  the  contrast  of 
brightness  between  the  sun-spots  and  the  photo- 
sphere decreases  towards  the  sun's  limb.  Langley, 
and  also  Frost,  found  indications  that  at  the  very 
limb  the  total  radiation  of  the  sun-spot  umbra  is  ac- 
tually stronger  than  that  of  the  photosphere.  W.  E. 
Wilson  observed  that  the  ratio  of  the  brightness  of 
the  spot  umbra  to  that  of  the  photosphere  at  the  sun's 
center  did  not  change  from  the  center  to  ninety-five 
per  cent  out  on  the  solar  radius,  whereas  the  ratio  of 
brightness  of  the  umbra  to  the  surroundings  increased 
from  T4A  to  T?OT-  He  could  not  confirm  Frost's  and 
Langley 's  result.  Schwartzchild  and  Villiger,  observ- 
ing at  wave-length  0.32/t  in  the  ultra-violet,  found  the 
ratio  of  brightness  of  sun-spots  to  the  surrounding 
photosphere  at  the  center  ten  to  fourteen  per  cent, 
but  close  to  the  limb  it  was  thirty  to  fifty  per  cent. 
It  has  already  been  stated  that  the  photosphere  at 
the  limb  of  the  sun  is  less  bright  than  it  is  at  the  cen- 

204: 


SUN-SPOTS,   FACUL.E,   AND   GRANULATION 

ter,  and  the  exact  amount  of  change  has  been  given 
for  various  wave  lengths  in  Chapter  III.  Accordingly 
it  is  easy  to  see  that  if,  as  observed  by  W.  E.  Wilson, 
the  sun-spot  umbra  remains  nearly  unchanged  in  its 
intrinsic  brightness  wherever  seen  upon  the  sun,  the 
results  just  mentioned  would  tend  to  follow.  It 
seems  hard  to  believe,  however,  that  the  radiation  of 
the  spot-umbra  at  the  limb  could  actually  exceed  that 
of  the  surrounding  photosphere,  as  observed  by  Frost 
and  Langley,  and  further  experiments  along  this  line 
should  be  made. 

(2)  In  sun-spot  spectra  many  Fraunhofer  lines  are 
strengthened  and  many  weakened  as  compared  with 
the  same  lines  in  the  photospheric  spectrum.1  From 
Adams'  summary  of  the  subject2 1  take  the  following 
data.  Calcium  has  sixty  lines  in  Rowland's  table  be- 
tween X  =  0.40//,  and  X  =  0.70/4,  and  with  one  possi- 
ble exception  all  are  strengthened  in  sun-spots.  The 
strengthening  increases  absolutely,  and  also  rela- 
tively to  the  intensities  of  the  lines  affected,  with  in- 
creasing wave  length.  With  iron  there  are  1 , 108  lines 
in  the  same  interval  of  Rowland's  table,  of  which  784 
are  affected  in  spots.  Of  these,  558  are  due  to  iron 

1  A  spectrum  absorption  line  is  said  to  be  strengthened  when,  by 
reason  of  its  becoming  broader  without  becoming  less  dark,  or  by 
reason  of  its  becoming  darker,  or  from  both  changes,  it  presents  a 
greater  contrast  to  the  adjoining  spectrum.     Weakening  a  spectrum 
line  implies  an  opposite  change.     In  either  case  the  term  is  relative, 
and  may  really  mean  the  alteration  of  the  adjoining  spectrum,  with- 
out change  in  the  line  itself,  in  such  a  manner  that  the  contrast  of  the 
line  is  altered. 

2  Contributions  of  the  Mount  Wilson  Solar  Observatory,  No.  40. 

205 


THE  SUN 


alone,  the  others  being  blends  of  iron  lines  with  very 
close  lines  of  other  elements.  Of  the  558  purely  iron 
lines  affected,  300  are  strengthened  and  258  weakened 
in  sun-spots.  Hydrogen  has  four  lines  in  the  region 
under  discussion,  and  all  are  weakened.  The  case  is  so 
striking  that  it  is  worth  giving  in  full: 

TABLE  XII. — Hydrogen  spectrum  in  sun-spots 


Line 

Wave  Length 

Intensity 

Photospheric 

Sun-spot 

Hs 

4101.848 
4340.471 
4861.350 
6562.835 

40N 
20N 
30 
40 

1 

4 
10 
25 

Hv 

Ha 

Ha                    

The  following  table  from  Adams'  publication  shows 
the  behavior  of  the  spot  lines  of  thirteen  different  ele- 
ments: 

TABLE  XIII. — Spectrum  lines  affected  in  sun-spots 


Element 

Total 

Number 
Lines 

Number  of 
Lines 
Strengthened 

Number  of 
Lines 
Weakened 

Percentage  of  Total 
Number 

One 
Ele- 
ment 

Com- 
pound 
Lines 
and 
Blends 

One 
Ele- 
ment 

Com- 
pound 
Lines 
and 
Blends 

Strength- 
ened 

Weak- 
ened 

Affec- 
ted 

Calcium  
Chromium.  .  . 
Cobalt  
Hydrogen  .  .  . 
Iron  
Magnesium  .  . 
Manganese  .  . 
Nickel  
Scandium.  .  . 
Silicon 

60 
386 
118 
4 
1108 
8 
167 
251 
45 
9 
8 
432 
176 

43 

200 
26 

300 
3 
68 
48 
30 

"s 

247 
114 

16 
75 
25 

i27 

'3i 
24 

'73 
37 

'36 
17 
4 
258 
1 
15 
106 
3 
8 

'46 
9 

3i 
14 

98 

'6 
26 

"i 

28 
5 

98 
71 
43 

39' 
38 
59 
29 
67 

100 
74 
86 

'l7 
26 
100 
32 
12 
14 
53 
7 
100 

'17 

8 

98 
88 
69 
100 
71 
50 
73 
82 
74 
100 
100 
91 
94 

Sodium  
Titanium  
Vanadium.  .  . 

206 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 

COOLNESS  OF  SUN-SPOTS 

If  the  layer  which  produces  the  Fraimhofer  lines 
over  the  spots  were  of  the  same  temperature  that  it  is 
over  the  photosphere,  the  lines  in  spots  would  tend  to 
appear  weakened;  because,  while  the  emission  in 
the  lines  would  in  that  case  remain  really  unchanged, 
the  spectrum  background  against  which  they  are  seen 
would  be  weakened,  and  approach  the  brightness  of 
the  lines,  as  has  been  seen  under  Caption  1.  Since  the 
reduction  of  the  background  in  sun-spot  spectra  is 
greatest  for  short  wave  lengths,  the  violet  lines  would 
be  most  weakened  in  the  case  we  are  considering. 
This  is,  indeed,  the  case  for  hydrogen,  and  may  be 
explained  in  that  case  perhaps  as  a  consequence  of 
high  level,  but  in  fact  the  majority  of  sun-spots  lines 
are  strengthened,  and  this  in  itself  may  be  regarded 
as  evidence  that  the  "reversing  layer"  for  most  ele- 
ments is  cooler  over  spots  than  over  the  photosphere. 
Besides  this  general  consideration,  there  are  several 
others,  now  to  be  mentioned,  which  point  to  the  same 
conclusion. 

Lines  which  are  relatively  stronger  in  the  electric 
spark  than  in  the  arc,  when  produced  as  bright  lines 
in  the  laboratory,  are  called  " enhanced  lines."  Of 
144  enhanced  lines  observed  in  spots,  says  Adams, 
"  130  are  distinctly  weakened,  none  are  strengthened, 
while  sixteen  show  no  marked  change."  This  almost 
universal  weakening  of  enhanced  lines  in  sun-spots 
is  shown  as  follows,  to  be  evidence  of  a  low  tempera- 

207 


THE  SUN 

ture  in  the  sun-spot  reversing  layer.  By  KirchhofFs 
law  (see  Chapter  II)  emission  and  absorption  are  pro- 
portional. Hence,  if  it  requires  the  conditions  of  the 
spark  to  produce  certain  emission  lines  strongly,  it 
will  also  require  the  conditions  of  the  spark  to  cause 
the  operative  gases  to  absorb  strongly  in  these  lines. 
But  spark  versus  arc  conditions  are  to  be  regarded  as 
of  high  versus  lower  temperatures,  a  view  fully  con- 
firmed by  the  experiments  of  Hale,  Adams,  and  Gale 
with  strong  and  weak  arcs,  and  those  of  King  with 
the  electric  furnace  at  high  and  low  temperatures. 
Accordingly,  the  weakening  of  the  enhanced  lines  in 
the  sun-spot  spectrum,  in  opposition  to  the  prevailing 
strengthening  of  lines  in  spots,  is  explained  by  assum- 
ing that  the  spot  vapors  are  too  cool  to  produce  strong 
absorption  of  enhanced  lines. 

A  third  line  of  evidence  showing  that  the  reversing 
layer  is  cooler  over  sun-spots  is  furnished  by  a  de- 
tailed comparison  of  the  spectra  of  sun-spots  and 
photosphere  on  the  one  hand,  and  of  low  and  high 
temperatures  in  the  arc  or  electric  furnace  on  the 
other.  This  comparison  was  begun  by  Hale,  Adams, 
.and  Gale,  and  continued  by  King.  Adams  gives  in  a 
long  table  the  results  of  such  a  comparison  for  the 
lines  of  iron.  From  this  table  several  of  the  most 
well-marked  cases,  typical  of  strengthening,  weaken- 
ing, and  neutrality,  are  given  in  the  following  table. 

In  general,  within  the  error  of  measurement,  lines 
strengthened  in  the  cool  arc  are  strengthened  in  sun- 
spots,  those  weakened  in  the  cool  arc  are  weakened  in 

208 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 


TABLE  XIV. — Sun-spot,  hot  arc  and  cool  arc  spectra 


Intensity 

Intensity 

Wave  Length 
(Rowland's) 

Spot 
ratio 

Arc  ratio 

Dis- 
crepancy 

Sun 

Spot 

Hot 
arc 

Cool 

arc 

4118.708 

5 

4 

16 

12 

1.2 

1.3 

—0.1 

4291.630 

2 

3 

8 

16 

0.7 

0.5 

+0.2 

4325.939 

8 

7 

48 

40 

1.1 

1.2 

—0.1 

4461.818 

4 

7 

19 

40 

0.6 

0.5 

+0.1 

4531.327 

5 

7 

16 

24 

0.7 

0.7 

+0.0 

4939.868 

3 

5 

10 

18 

0.6 

0.6 

0.0 

5083.518 

4 

6 

12 

22 

0.7 

0.5 

+0.2 

5202.516 

4 

4 

14 

16 

1.0 

0.9 

+0.1 

5333.089 

4 

7 

7 

16 

0.6 

0.4 

+0.2 

5405.989 

6 

10 

40 

80 

0.6 

0.5 

0.1 

6024.281 

7 

7 

13 

13 

1.0 

1.0 

0.0 

sun-spots,  and  those  unchanged  in  one  are  unchanged 
in  the  other,  and  all  by  similar  proportions.  It  fol- 
lows from  this,  by  a  similar  line  of  argument  to  that 
just  given  for  enhanced  lines,  that  the  reversing  layer 
is  relatively  cooler  over  sun-spots  than  over  the  pho- 
tosphere. 

A  fourth  phenomenon  strongly  indicating  the  same 
conclusion  is  the  highly  conspicuous  presence  in  sun- 
spot  spectra  of  flutings,  or  rythmic  banded  appear- 
ances, immensely  numerous,  and  characteristic  re- 
spectively of  the  spectra  of  titanium  oxide,  magnes- 
ium hydride,  and  calcium  hydride.  The  identifica- 
tions of  these  flutings  were  discovered  respectively 
by  Hale,  Adams,  and  Gale,  by  Fowler  and  by  Olm- 
sted.  These  and  other  molecular  compounds  give,  as 
Evershed  has  stated,  very  slight  and  not  always  per- 

209 


THE  SUN 

ceptible  evidence  of  their  presence  in  the  photo- 
spheric  spectrum.  It  is  well  known  that  high  tem- 
peratures tend  to  produce  complete  dissociation  of 
molecular  compounds.  The  copious  appearance  of 
the  lines  of  compounds  in  the  spectra  of  sun-spots 
would  be  very  strong  evidence  of  the  relatively  low 
temperature  in  the  reversing  layer  above  spots,  even 
if  unsupported  by  the  other  evidences  given  above, 
and  by  many  other  minor  phenomena  of  which  space 
forbids. the  mention. 

According  to  Father  Cortie,1  steam  also  occurs  in 
sun-spots,  for  he  finds  water- vapor  lines  among  those 
widened  in  sun-spot  spectra.  He  cites  experiments, 
too,  which  indicate  that  the  spectrum  of  magnesium 
hydride  could  not  show  in  sun-spots  if  water  vapor 
was  not  also  present.  Evershed,  however,  concludes 
from  observations  at  the  high  and  dry  station  of 
Kodaikanal  that:  "On-  the  whole,  it  must  be  ad- 
mitted that  the  evidence  for  the  strengthening  of 
telluric  lines,  of  whatever  origin,  in  spot  spectra  is 
practically  negligible." 

An  excellent  photographic  map  of  the  sun-spot 
spectrum,  contrasted  with  that  of  the  photosphere, 
has  been  prepared  at  the  Mount  Wilson  Solar  Ob- 
servatory and  distributed  to  solar  observers.  Plate 
XVII,  reproduced  here  by  the  permission  of  the 
Director,  shows  a  section  of  this  map  including  the 
b  group.  Although  no  engraving  can  do  full 
justice  to  the  original,  the  reader  will  be  able  to 

1Astrophysical  Journal,  vol.  xxviii,  p.  379,  1908. 
210 


SUN-SPOTS,   FACUL.E,   AND   GRANULATION 

note   for  himself  some  of  the  features  mentioned 

above. 

SUN-SPOTS  AND  MAGNETISM 

In  the  year  1908,  Hale  discovered  the  existence  of  a 
magnetic  field  in  spots,  which  betrays  its  presence  by 
the  widening,  doubling,  or  tripling  of  a  great  number 
of  spectral  lines.  As  stated  in  Chapter  II,  Zeeman 
discovered,  about  1896,  that  most  lines  of  the  spec- 
trum are  separated  into  two  components  when  viewed 
along  the  lines  of  force  of  a  powerful  magnet,  and  the 
two  components  are  circularly  polarized  in  opposite 
directions.  With  less  powerful  fields,  the  lines  are 
not  clearly  doubled,  only  widened,  but  their  right- 
and  left-hand  edges  exhibit  in  this  case  traces  of  op- 
posite circular  polarization.  Hale  applied  this  test 
of  polarization  to  the  most  widened  lines  of  sun-spots 
by  introducing  a  Fresnel  rhomb  to  convert  the  sup- 
posed circular  to  plane  polarization,  and  found  the 
right-hand  or  left-hand  edge  of  the  lines  could  be  cut 
off  at  will,  according  to  the  position  of  the  Nicol 
prism  used  for  analyzing  the  character  of  polarization 
of  the  light.  Some  lines  are  triple  in  spots,  tjjit  these 
seeming  discrepancies  proved  to  be  the  best  of  evi- 
dence of  the  effect  of  a  magnetic  field.  For  when  the 
same  lines  were  examined  in  the  laboratory  they 
proved  exceptional,  and  to  become  triple  instead  of 
double  when  viewed  along  the  magnetic  lines  of  force. 
Hale's  brilliant  discovery  has  cleared  up  one  of  the 
most  puzzling  questions  relating  to  the  sun-spot 
spectrum. 


THE  SUN 

By  polarization  studies,  Hale  found  that  sun-spot 
fields  are  not  always  of  the  same  polarity.  Very 
often  a  pair  of  sun-spots  quite  near  together  are  found 
to  be  of  opposite  polarity.  In  general,  the  polarity 
of  spots  in  the  sun's  southern  hemisphere  is  opposite 
to  that  in  the  northern,  but  there  are  very  numerous 
exceptions  to  this  rule,  as,  of  course,  in  the  case  of 
double  spots,  as  just  mentioned.  Spots  near  the  sun's 
limb,  since  they  present  their  magnetic  lines  of  force 
nearly  at  right  angles  to  our  line  of  sight,  tend  to  show 
triple  lines  where  doublets  would  be  seen  near  the 
center  of  the  disk. 

The  cause  of  the  magnetic  field  in  sun-spots  is  a 
most  interesting  problem.  Rowland  showed  many 
years  ago  that  static  electric  charges,  in  rotation, 
produce  electro-magnetic  effects  similar  to  those  pro- 
duced by  electric  currents  in  coils  of  wire.  This  seems 
to  point  the  way  to  a  solution,  for,  as  stated  in  the 
account  of  the  spectroheliographic  results  in  Chapter 
III,  the  sun  when  viewed  through  the  hydrogen  line 
Ha  (C)  shows  curved  formations  (see  Plate  XI), 
which  seem  to  indicate  spiral  motion  in  sun-spot 
neighborhoods.  In  such  Ha  photographs  of  double 
spots,  which  give  opposing  magnetic  polarity,  the 
curves  which  surround  the  spots  seem  to  present  the 
appearance  not  unlike  those  seen  among  iron  filings 
on  a  sheet  of  paper  acted  upon  by  a  pair  of  opposite 
magnetic  poles.  It  seems,  then,  not  improbable 
that  whirling  motions  or  vortices  exist  in  sun-spots, 
and  that  these  carry  along  electrically  charged  par- 

212 


SUN-SPOTS,   FACUL.E,   AND  GRANULATION 

tides  which  produce  the  observed  magnetic  fields. 
The  impression  was  at  first  that  these  charges  were 
the  so-called  ions,  or  bodies  smaller  than  atoms,  re- 
cently made  known  by  J.  J.  Thomson  arid  others;  but 
great  difficulty  was  found  in  accounting  for  their 
isolation  in  sun-spots  in  sufficient  numbers.  It  was 
suggested  to  Mr.  Hale  by  the  writer  that  the  mole- 
cules of  the  compounds  shown  in  the  sun-spot  spec- 
trum, or  perhaps  even  the  relatively  cooled  elemen- 
tary gases  in  spots,  might  very  probably  be  regarded 
as  sufficiently  different  from  the  surroundings  to  pro- 
duce frictional  electricity,  when  whirled  about  in 
the  spots,  just  as  steam  becomes  electrified  in  Arm- 
strong's machine  when,  carrying  water-droplets,  it 
issues  from  an  orifice.  Further  discussion  of  the 
matter  will  be  found  in  Chapter  VI. 

RADIAL  MOTION  IN  SPOT  PENUMBRAS 

Evershed  has  lately  observed  shifting  of  spectral 
lines  in  the  penumbras  of  spots  situated  at  consider- 
able distance  from  the  center  of  the  sun's  limb.  This 
seems  to  indicate  motion  nearly  radial  to  the  center 
of  the  spots,  as  if  material  was  coming  to  the  sun's 
surface  in  the  sun-spot  centers,  and  then  spreading 
out  in  all  directions,  like  smoke  from  a  volcano. 
Nevertheless,  no  spectroscopic  evidence  of  motion  in 
spots  radial  to  the  center  of  the  sun  has  ever  been  ob- 
tained.1 Adams  has  lately  sought  to  find  evidences 

1  As  this  is  being  published  St.  John  has  observed  high  level  gases 
moving  downwards  in  spots. 

213 


THE  SUN 

of  increased  or  decreased  pressure  in  the  reversing 
layer  over  sun-spots,  from  shifting  of  lines  known  to 
be  subject  to  large  shifts  when  their  sources  are  under 
pressure,  but  he  was  unable  to  discover  evidences  of 
altered  pressure.  The  significance  of  these  facts  will 
be  discussed  in  Chapter  VI. 


CHAPTER  VI. 

WHAT   IS   THE    SUN? 

Young's   Views. — Halm's   Views. — Schmidt's    Hypothesis. — Julius' 
Views. — The  Author's  Views. 

BELIEVING  that  the  views  of  the  late  Professor 
Young  probably  are  still  shared  by  a  majority  of  as- 
tronomers, even  after  the  lapse  of  fifteen  years  since 
the  appearance  of  the  last  revision  of  his  work,  "The 
Sun, "  we  shall  begin  this  chapter  by  quoting  a  part  of 
the  summary  which  he  gives  in  his  Chapter  IX.  We 
shall  then  take  up  the  solar  theories  of  Halm,  Schmidt, 
and  Julius.  In  the  remainder  of  the  chapter  we  shall 
consider  still  another  view  of  the  matter,  which  the 
present  writer  inclines  to  adopt. 

YOUNG'S  VIEWS 

Quoting  from  Young's  "The  Sun:" 

"Fig.  56  is  intended  to  present  to  the  eye,  more 
clearly  than  any  mere  description,  the  constitution  of 
the  sun,  and  the  relation  of  the  different  concentric 
shells  or  envelopes  as  conceived  by  the  writer. 

"The  picture  is  an  ideal  section  through  the  center. 
The  black  disk  represents  the  inner  nucleus,  which  is 
not  accessible  to  observation,  its  nature  and  constitu- 
tion being  a  mere  matter  of  inference.  The  white 
16  215 


THE  SUN 


ring  surrounding  it  is  the  photosphere,  or  shell  of  in- 
candescent cloud  which  forms  the  visible  surface. 
The  depth,  or  thickness,  of  this  shell  is  quite  un- 
known; it  may 
be  many  times 
thicker  than  rep- 
resented, or 
possibly  some- 
what thinner. 
Nor  is  it  certain 
whether  it  is 
separated  from 
the  inner  core 
by  a  definite  sur- 
face, or  whether, 
on  the  other 
hand,  there  is  no 
distinct  bound- 
ary between 
them. 

"  The  outer 
surface  of  the 
photosphere, 
however,  is  cer- 
tainly pretty 
sharply  defined,  though  very  irregular,  rising  at  points 
into  faculse,  and  depressed  at  others  in  spots,  as 
shown  in  the  figure. 

"  Immediately  above  this  lies  the  so-called  '  revers- 
ing stratum, '  in  which  the  Fraunhofer  lines  originate. 

216 


FIG.  56. — SOLAR  DIAGRAM.    (Young.) 


WHAT  IS  THE  SUN? 

It  is  to  be  noted,  however,  that  the  gases  which  com- 
pose this  stratum  do  not  merely  overlie  the  photo- 
sphere, but  they  also  fill  the  interspaces  between  the 
photospheric  clouds,  forming  the  atmosphere  in 
which  they  float,  and  an  attempt  has  been  made  to 
indicate  this  fact  in  the  diagram. 

"  Above  the  '  re  versing  stratum7  lies  the  scarlet 
chromosphere,  with  prominences  of  various  forms  and 
dimensions  rising  high  above  the  solar  surface;  and 
over,  and  embracing  all,  is  the  coronal  atmosphere 
and  the  mysterious  radiance  of  clouds,  rifts,  and 
streamers,  fading  gradually  into  the  outer  darkness. 

"At  the  center  of  the  sun  the  earth  is  represented  in 
its  true  relative  dimensions — TTO  of  the  three  inches 
which  is  taken  as  the  scale  of  the  sun's  diameter.  This 
scale  reduces  our  globe  to  a  little  dot  only  ^B-  of  an 
inch  across.  Around  it,  at  its  proper  distance,  is 
drawn  the  orbit  of  the  moon,  still  far  within  the  pho- 
tosphere, the  moon  herself  being  fairly  represented 
by  any  one  of  the  minute  points  which  make  up 
the  dotted  line  that  indicates  her  path. 

"The  central  nucleus  is  made  black  in  the  picture, 
simply  for  convenience,  and  not  with  any  purpose  to 
indicate  that  the  matter  which  composes  it  is  cooler  or 
even  less  brilliantly  luminous  than  the  photosphere. 
It  is  quite  probable,  indeed,  that  this  central  core 
(which  contains  certainly  more  than  nine-tenths  of 
the  whole  mass  of  the  sun)  is  purely  gaseous,  and  it  is 
of  course  true  that,  at  a  given  temperature  and  pressure, 
a  gaseous  mass  has  a  lower  radiating  power,  and  is 

217 


THE  SUN 

less  luminous,  than  a  mass  of  clouds,  such  as  those 
which  constitute  the  photosphere.  But,  on  the  other 
hand,  both  compression  and  increase  of  temperature 
rapidly  raise  the  radiating  power  of  a  gas;  and  it  is 
highly  probable  that,  at  no  very  considerable  depth, 
the  growing  pressure  and  heat  may  more  than  equal- 
ize matters,  and  render  the  central  nucleus  as  in- 
tensely bright  as  the  surface  of  the  sun  itself. 

"  At  the  upper  surface  of  the  photosphere,  however, 
and  all  through  it,  indeed,  the  uncondensed  gases  are 
dark  as  compared  with  the  droplets  and  crystals 
which  make  up  the  photospheric  clouds.  Here  the 
pressure  and  temperature  are  lowered,  so  that  the 
vapors  give  out  no  longer  a  continuous  but  a  bright- 
line  spectrum,  whenever  we  get  a  chance  to  see  them, 
against  a  non-luminous  background;  and,  when  the 
intenser  light  from  the  liquid  and  solid  particles  of  the 
photosphere  shines  through  these  vapors,  they  rob  it 
or  the  corresponding  rays,  and  produce  for  us  the 
familiar  dark-lined  spectrum  of  ordinary  sunlight. 

"  Although  it  may  not  be  possible,  in  the  present 
state  of  science,  to  demonstrate  that  the  principal  por- 
tion of  the  solar  mass  is  gaseous,  this  much  can  at  least 
be  said — that  a  globe  of  incandescent  gas,  under  condi- 
tions such  as  have  been  intimated,  would  necessarily 
present  just  such  phenomena  as  the  sun  exhibits. 

"On  the  outer  surface,  exposed  to  the  cold  of  space, 
the  rapid  radiation  would  certainly  produce  the  con- 
densation and  precipitation  into  luminous  clouds  of 
such  vapors  as  had  a  boiling-point  higher  than  that  of 

218 


WHAT   IS   THE   SUN? 

the  cooling  surface.  These  clouds  would  float  in  an 
atmosphere  saturated  with  the  vapors  from  which 
they  were  formed,  and  also  containing  such  other  va- 
pors as  were  not  condensed,  and  thus  the  peculiarities 
of  the  solar  spectrum  would  result.  On  the  other 
hand,  the  permanent  gases,  like  hydrogen — those  not 
subject  to  condensation  into  the  liquid  form  under 
the  solar  conditions — would  rise  to  higher  elevations 
than  the  others,  and  form  above  the  photosphere  just 
such  a  chromosphere  as  we  observe.  Whether,  from 
the  mere  assumption  of  such  a  constitution  for  the 
sun,  one  could  work  out,  a  priori,  the  phenomena  of 
sun-spots  and  prominences,  is  indeed  doubtful;  but 
thus  far  nothing  in  any  of  them  has  been  observed 
which  appears  to  be  inconsistent  with  this  view  of  the 
subject — nothing,  we  say,  unless  it  should  turn  out,  as 
was  once  maintained,  that  the  solar  surface  possesses, 
so  to  speak, '  geographical '  characteristics,  evinced  by 
the  disposition  to  break  out  into  sun-spots  at  certain 
fixed  points — as  if  at  those  points  there  were  volca- 
noes or  something  of  the  sort.  Of  course,  the  fact  that 
the  spots  are  distributed  mainly  in  two  belts  parallel 
to  the  solar  equator,  involves  no  difficulty,  for  it  is 
easy  to  conceive  how,  in  more  than  one  way,  the  sun's 
rotation  might  lead  to  such  a  result :  but  peculiarities 
permanently  attaching  to  individual  points  on  the 
solar  surface  necessarily  imply  rigid  connections,  such 
as  are  inconsistent  with  the  theory  of  a  gaseous  or 
even  of  a  fluid  nucleus.  But  while,  as  has  been 
already  pointed  out,  there  is  a  marked  tendency  in 

219 


THE  SUN 

spots  to  recur  at  or  near  the  same  points  during  sev- 
eral solar  revolutions,  there  is  no  evidence  which  es- 
tablishes the  existence  of  fixed  spot-centers;  and  the 
idea  is  to  be  regarded  merely  as  a  relic  of  the  old 
Herschellian  theory  of  a  solid  sun.  Still  it  is  difficult 
to  test  the  notion  conclusively  even  by  means  of  such 
extended  observations  as  those  of  Carrington  or 
Spoerer,  or  the  auroral  periods  of  Veeder,  since  the 
time  of  rotation  of  the  solid  nucleus,  if  it  exists  at  all, 
is  unknown,  and  this  makes  the  discussion  difficult 
and  unsatisfactory. 

"With  reference  to  the  constitution  of  the  photo- 
sphere there  is  a  general  agreement  among  astrono- 
mers. A  few,  perhaps,  still  hold,  as  has  been  men- 
tioned, to  the  idea  that  the  visible  surface  is  a  liquid 
sheet,  w^hile  some  believe  that  it  is  purely  gaseous; 
but  the  whole  appearance  of  things,  the  details  of  the 
granulation,  the  phenomena  of  spots  and  faculse,  the 
mobility  and  variability  of  the  floccules,  all  better 
accord  with  the  theory  adopted  in  these  pages,  which 
is  a  necessary  consequence  of  the  hypothesis  that  the 
sun  is  principally  gaseous.  It  seems  almost  impos- 
sible to  doubt  that  the  photosphere  is  a  shell  of  clouds. 
As  to  the  precise  constitution  of  this  shell,  however, 
the  form  and  magnitude  of  the  component  cloudlets, 
the  chemical  elements  involved,  and  the  temperature 
and  pressure,  there  is  room  for  a  good  deal  of  uncer- 
tainty and  difference  of  opinion.  The  more  common 
view,  apparently — the  one,  certainly,  which  the 
writer  has  hitherto  held — is,  that  the  clouds  are 

220 


WHAT  IS  THE  SUN? 

formed  mainly  by  the  condensation  of  the  substances 
which  are  most  conspicuous  in  the  solar  spectrum, 
such  as  iron  and  the  other  metals.  As  to  the  form  of 
the  clouds,  also,  it  has  usually  been  assumed  that, 
as  a  consequence  of  the  ascending  currents  by  which 
they  are  formed,  they  are  columnar,  their  height 
being  much  greater  than  their  other  dimensions. 

"  Professor  Hastings  has  proposed  a  somewhat  dif- 
ferent theory,  which  avoids  some  of  the  difficulties  of 
the  received  doctrine,  though  not  without  encounter- 
ing others  which  seem  just  as  formidable. 

"One  main  peculiarity  is  the  assumption  that  the 
photospheric  '  clouds '  are  formed  by  the  precipitation 
of  either  carbon,  silicon,  or  boron  (the  three  members 
of  the  carbon  group),  to  the  exclusion  of  other  sub- 
stances which  are  less  refractory  (have  lower  boiling- 
points),  and  therefore  escape  precipitation. 

"His  idea  that  the  stratum  which  produces  the  gen- 
eral absorption  at  the  limb  of  the  sun  is  a  veil  of 
'smoke' — i.  e.,  of  the  same  minute  particles  which 
constitute  the  photosphere,  but  cooled  to  relative 
darkness — has  been  already  alluded  to  in  a  preceding 
chapter.  So  far  as  we  know,  it  is  novel  and  valuable, 
clearing  up  a  good  many  embarrassing  difficulties.  It 
is  so  obvious,  on  reflection,  that  something  of  the  sort 
must  accompany  the  photosphere,  that  it  is  surprising 
that  the  idea  had  not  been  thought  of  before.  Of 
course,  the  particles  formed  by  condensation  must, 
many  of  them  at  least,  be  carried  by  the  ascending 
currents  high  above  the  point  of  their  formation,  and 

221 


THE  SUN 

cooled  so  much  as  to  become  relatively  dark  in  com- 
parison with  the  more  vivid  incandescence  of  the  re- 
gions below,  just  as  the  ascending  particles  of  carbon, 
unconsumed  and  cooled,  constitute  the  smoke  of  a  fire. 

"The  idea  that  carbon  may  be  the  main  constitu- 
ent of  the  photosphere  is  by  no  means  new:  it  was  first 
seriously  advanced,  we  believe,  by  Johnstone  Stoney, 
of  Dublin,  as  early  as  1867,  mainly  on  physico-chem- 
ical grounds,  and  is  enthusiastically  advocated  by  Sir 
Robert  Ball  in  his  recent '  Story  of  the  Sun. ' 

"As  regards  the  'reversing  stratum'  very  little  need 
be  added.  Mr.  Lockyer  indeed  denies  its  existence — 
that  is,  in  the  sense  that  there  is  a  thin  stratum,  close 
above  the  surface  of  the  photosphere,  in  which  most 
of  the  dark  lines  of  the  solar  spectrum  originate.  He 
maintains,  on  the  contrary,  in  accordance  with  his 
'  dissociation  theory, '  that  certain  of  the  lines,  due  to 
substances  the  most  nearly  elementary,  and  having 
their  molecules  in  the  highest  stage  of  dissociation, 
originate  only  deep  down  in  the  solar  atmosphere 
where  the  heat  is  most  intense :  others,  due  to  vapors 
with  molecules  somewhat  less  simple,  have  their  birth 
a  little  higher;  and  others  yet,  due  to  molecules  the 
most  complex,  are  produced  only  in  the  most  elevated 
regions  of  the  solar  atmosphere;  each  elevation  thus 
being  responsible  for  its  own  special  family  of  spec- 
trum lines. 

" If,  however,  we  reject  this  theory  as  'not  proven, ' 
we  get  results  not  very  different. 

"The  vapors  of  the  photosphere  and  chromosphere 


WHAT  IS  THE  SUN? 

are  not  to  be  thought  of  as  entirely  separate  and  dis- 
tinct. All  the  gases  are  found  together  in  the  inter- 
stices between  the  cloud-granules  of  the  photosphere 
—the  unknown  substance  which  produces  the  green 
line  in  the  spectrum  of  the  corona,  the  hydrogen,  the 
calcium,  and  helium  which  characterize  the  chromo- 
sphere, and  the  metallic  vapors  which  give  the  re- 
versing layer  its  peculiar  properties — these  all  exist 
together  in  the  lower  depths,  unless,  indeed,  it  may 
possibly  be  the  case  that  at  the  greater  elevations 
some  compound  bodies  are  formed  which  can  not  exist 
in  the  fiercer  fires  below.  So  far  as  we  can  distinguish 
between  these  different  portions,  we  may  define  the 
photosphere  as  the  shell  within  which  precipitation  is 
taking  place;  the  reversing  layer,  as  that  lowest  re- 
gion of  the  solar  atmosphere  which  contains  sensibly 
all  the  gases  indicated  by  the  spectroscope ;  the  chro- 
mosphere, as  the  region  of  hydrogen,  calcium,  and 
helium;  and  the  corona,  as  that  upper  domain  of  the 
solar  atmosphere  which  becomes  observable  only  dur- 
ing solar  eclipses.  But  the  coronal  gas  itself  is  most 
conspicuous  and  abundant  right  in  the  photosphere 
and  reversing  layer,  and  the  same  is  true  of  the  hydro- 
gen of  the  prominences. 

"It  is  well,  also,  to  bear  in  mind  that,  if  any  sub- 
stances decomposable  by  heat  exist  upon  the  sun  at 
all,  we  must  expect  to  find  them  in  the  higher  and 
cooler  regions  of  the  solar  atmosphere.  In  and  near 
the  photosphere,  or  underneath  it,  matter  must  be  in 
in  its  most  elemental  state. 


THE  SUN 

"As  to  the  mechanism  of  the  chromosphere  and 
prominences,  if  we  may  use  the  expression,  much  cer- 
tainly remains  to  be  learned.  In  many  cases,  indeed, 
perhaps  in  most,  the  forms  and  behavior  of  the  protu- 
berances are  satisfactorily  enough  accounted  for  by 
supposing  that  the  heated  hydrogen  and  its  associate 
vapors  is  simply  forced  up  into  cooler  regions  by  pres- 
sure from  below — a  pressure  which  must  result  from 
the  downward  movement  of  the  great  mass  of  pre- 
cipitated matter  which  forms  the  photosphere.  But 
evidently  this  is  not  the  whole  story.  We  must  have 
recourse  to  ideas  of  a  different  order  to  account  for  the 
somewhat  rare,  but  still  really  numerous  and  well- 
authenticated  instances  when  the  summits  of  prom- 
inences have  been  seen  to  rise  in  a  few  minutes  to  ele- 
vations of  two  or  three  hundred  thousand  miles,  the 
upward  motion  being  almost  visible  to  the  eye  at  the 
rate  of  a  hundred  miles  a  second  or  more. 

"Very  perplexing,  also,  is  the  indubitable  fact  that 
clouds  of  this  prominence-matter  sometimes  gather 
and  form  without  any  apparent  connection  with  the 
chromosphere  below,  apparently  just  as  clouds  form 
in  our  own  atmosphere,  by  the  condensation  of  vapor 
before  invisible.  On  the  whole,  it  looks  very  much  as 
if  we  must  regard  the  prominences  as  differing  from 
the  surrounding  medium  mainly,  if  not  wholly,  in 
their  luminosity — as  simply  superheated  portions  of 
an  immense  atmosphere. 

"But,  then,  we  immediately  encounter  the  difficul- 
ties so  ably  urged  by  Lane,  Lockyer,  and  others,  that 

224; 


WHAT  IS  THE  SUN? 

the  existence  of  hydrogen  of  any  appreciable  density, 
at  the  elevation  of  even  a  hundred  thousand  miles, 
implies  a  density  and  pressure  at  the  surface  of  the 
photosphere  so  high  as  to  be  entirely  inconsistent  with 
the  spectroscopic  phenomena  there  manifested — un- 
less, indeed,  under  solar  conditions,  the  action  of 
gravity  upon  the  gases  of  the  solar  atmosphere  is 
modified  by  some  repulsive  force.  That  such  a  force 
is  at  least  conceivable,  is  obvious  from  the  behavior 
of  the  tails  of  comets;  and  many  features  in  the  cor- 
ona point  in  the  same  direction.  Of  its  nature  and 
origin  we  can  not,  however,  assert  anything  as  yet. 

"Even  more  difficult  than  the  problem  of  the 
chromosphere  is  that  of  the  corona.  While  it  is  some- 
thing to  know  that  the  phenomenon  is  mainly  solar, 
and  that,  therefore,  it  must  rank  in  magnitude  and 
importance  with  the  most  magnificent  of  natural  ob- 
jects, we  have  yet  to  find  a  satisfactory  explanation 
of  many  of  its  most  obvious  features.  It  is  certainly 
very  complex — matter  meteoric  and  matter  truly 
solar;  orbital  motion,  solar  attraction,  atmospheric 
resistance,  and  actions  thermal,  electrical,  and  mag- 
netic, are  probably  all  combined." 

HALM'S  VIEWS 

Since  the  time  when  Young  wrote,  Halm  has  con- 
tributed the  following  theory,  designed  particularly 
to  explain  the  periodicity  of  sun-spots.1  Halm  calls 

1  Annals  Royal  Observatory  Edinburg,  vol.  i,  pp.  74-151, 
1902. 

225 


THE  SUN 

attention  to  the  function  of  the  so-called  solar  enve- 
lope, and  refers  to  the  views  of  Langley,  Pickering, 
and  others,  that  it  prevents  the  escape  of  half  of  the 
solar  energy.  He  then  considers  the  effect  of  changes 
in  its  powers  of  restraint.  He  accepts  Helmholtz's 
hypothesis  that  the  source  of  the  solar  energy  lies  in 
the  contraction  of  the  sun,  and  thinks,  in  contradic- 
tion to  See's  views,  that  the  sun  is  already  gradually 
cooling.  He  then  refers  to  Hastings'  paper  (cited  by 
Young)  on  the  nature  of  the  solar  envelope,  and  says : 
"  Indeed,  it  seems  obvious  that  these  particles  which, 
while  ascending  from  the  interior  to  the  surface,  are 
precipitated  so  as  to  form  the  luminous  clouds  of  the 
photosphere  must  (quoting  Hastings)  'rapidly  cool 
on  account  of  their  great  radiating  power,  and  form  a 
fog  or  smoke  which  settles  slowly  through  the  spaces 
.between  the  granules'  and  that  'it  is  this  smoke  which 
produces  the  general  absorption  at  the  limb. ' ' '  Then, 
to  emphasize  the  importance  of  heat  conservation  by 
the  solar  envelope,  Halm  refers  to  Langley 's  early 
view  (which  apparently  Halm  has  not  noticed  that 
Langley  afterwards  retracted)  to  the  effect  that  the 
earth's  temperature  would  fall  to  -200°  C.  if  it  had 
no  atmosphere.1 

He  suggests  that  if  gravitation  should  be  temporar- 
ily too  little  to  supply  heat  energy  by  contraction  to 
balance  lost  energy  of  radiation,  then  the  layer  of 

1  See  "Report  of  the  Mt,  Whitney  Expedition"  p.  123,  and  "The 
Temperature  of  the  Moon,"  Memoirs  National  Academy,  vol.  iv, 
pt.  2,  p.  193. 

226 


WHAT  IS   THE  SUN? 

maximum  incandescence  would  cool,  and  a  lower 
layer  would  become -the  new  layer  of  maximum  in- 
candescence. The  absorbing  layer  thereby  increases  in 
thickness,  so  that  the  new  layer  of  maximum  incan- 
descence dissipates  less  energy  than  the  first.  Thus, 
at  length  a  layer  is  reached  which  dissipates  energy  of 
radiation  as  fast  as  gravitation  supplies  energy  of  heat. 
But,  when  this  state  occurs,  the  outer  layers  will  still 
go  on  cooling,  since  they  receive  less  radiation  from 
within  than  formerly.  Consequently  they  continue  to 
grow  more  opaque,  and  the  amount  of  energy  of  radi- 
ation dissipated  to  space  thereby  becomes  less  than 
the  amount  of  heat  energy  supplied  by  contraction. 

Halm  continues:  "It  thus  comes  to  pass  that, 
while  the  function  of  the  absorbing  envelope  is  that 
of  reducing  as  much  as  possible  the  waste  of  energy 
from  the  photospheric  layers  beneath,  it  is,  by  the 
very  nature  of  the  process,  compelled  to  overdo  its 
work,  and  to  finally  preserve  too  much  energy  within 
the  star.  The  outbreak  of  eruptions  and  the  forma- 
tion of  spots  are  the  consequence  of  an  unstable  equi- 
librium in  the  photospheric  layers,  and  take  place 
whenever  the  supply  of  heat  from  the  interior  is  so 
supplemented  by  the  continuous  reflection  of  heat 
from  the  overlying  atmosphere  that  the  photospheric 
layers  receive  more  heat  than  is  required  for  the  main- 
tenance of  their  thermal  equilibrium. 

"The  function  of  eruptions,  consisting  as  they  do 
in  the  ejection  of  overheated  photospheric  matter,  is 
to  produce  a  general  heating  and  clearing  up.  of  the 


THE  SUN 

cooled  absorbing  layers  of  the  solar  envelope.  The 
action  of  the  spots  consists  in  drawing  the  cooled  por- 
tions of  this  atmosphere  into  the  hotter  regions  of  the 
photosphere/' 

Halm  then  goes  on  with  mathematical  work  aimed 
to  show  that  the  consequences  of  these  principles  lead 
to  a  periodicity  of  sun-spot  phenomena  similar,  even 
in  its  details,  to  that  actually  observed,  but  this  part 
of  the  paper  does  not  seem  to  be  so  soundly  based  as 
to  add  much  to  the  merit  of  his  views. 

There  is  a  strong  objection  to  Halm's  mathematical 
analysis,  which  applies,  also,  to  the  several  computa- 
tions made  by  Vogel,  Pickering,  and  others  to  deter- 
mine the  effectiveness  of  the  so-called  "solar  enve- 
lope," which  these  astronomers  regard  as  a  layer 
which  restrains  the  emission  of  the  sun.  For  they 
treat  only  the  losses  suffered  by  the  direct  beam 
through  scattering  in  this  envelope,  without  taking 
account  of  the  gains  which  the  beam  acquires  from 
rays  scattered  into  it  by  the  same  envelope.  Their 
numerical  results  are  hence  of  no  application  to  the 
sun ;  for  the  light  proceeding  in  a  single  direction 
from  any  point  in  the  "envelope"  is  derived  from 
almost  a  full  hemisphere.  Their  formulae  are  appli- 
cable only  to  a  case  like  that  of  the  earth's  atmos- 
phere, where  the  entering  rays  are  practically  all 

parallel. 

SCHMIDT'S  HYPOTHESIS 

It  was  in  1891  that  Schmidt  published  his  theory  of 
a  gaseous  photosphere,  and  he  explained  the  appar- 

'  228 


WHAT  IS  THE  SUN? 

cntly  sharp  outline  of  the  sun  in  a  very  ingenious  and 
interesting  way.  It  is  well  known  that  the  sun  and 
other  objects  are  seen,  after  they  get  below  the  real 
horizon  of  the  earth,  by  the  refraction  of  the  air, 
which  curves  the  rays  of  light.  The  amount  of  curva- 
ture depends  on  the  rate  of  change  of  optical  density 
of  the  atmosphere  from  its  outer  limits  to  the  surface 
of  the  earth.  At  sea-level  the  difference  caused  by 
refraction  between  the  apparent  and  real  positions  of 
heavenly  bodies  is  about  one-half  a  degree  of  arc. 
Suppose  the  earth  were  to  grow  larger,  but  with  the 
same  atmospheric  densities  prevailing.  There  would 
be  a  certain  limiting  diameter,  about  seven  times  that 
of  the  earth,  where  the  curvature  of  the  rays  would  be 
just  sufficient  to  cause  them  to  bend  entirely  around 
the  earth  in  passing  from  the  top  to  the  bottom  of  the 
atmosphere,  so  that  if  there  were  no  loss  of  light  on 
the  circuit  a  man  as  tall  as  the  atmosphere  is  thick 
might  be  imagined  to  stand  on  his  head  at  the  equa- 
tor, and  looking  directly  in  front  of  him  see  his  own 
heels  all  the  way  around  the  world.  If  the  earth  were 
supposed  still  larger  then  all  the  rays  leaving  its  sur- 
face tangentially  would  be  incurved,  and  reach  the 
surface  again  at  some  other  point,  without  ever  suc- 
ceeding in  escaping  to  space. 

Schmidt  conceived  of  the  sun  as  a  wholly  gaseous 
body,  above  the  limiting  size  just  discussed.  Accord- 
ingly, looking  from  the  earth,  there  would  be  a  cer- 
tain diameter  at  which  the  line  of  sight  would  curve 
around  in  an  infinitely  long  spiral  of  practically  con- 

229 


THE  SUN 

slant  diameter.  A  line  of  sight  outside  of  this  would 
pass  nearly  straight  through  the  outer  layers  of  gas, 
and  emerge  on  the  opposite  side  of  the  sun  in  space. 
A  line  of  sight  to  a  smaller  circumference  would  pass 
along  a  diminishing  spiral  inside  the  sun  till  it  almost 
reached  the  lesser  sphere,  to  which  it  would  finally  be 
tangent;  and  there  it  would  go  around  and  around  in 
an  infinite  spiral  course  of  practically  constant  diam- 
eter. Hence,  all  lines  of  sight  inside  a  certain  limiting 
circumference  would  give  brilliant  effects  because 
they  would  have  an  infinitely  long  path  of  incandes- 
cent gas  of  great  density-  to  take  light  from;  while  all 
lines  of  sight  outside  this  limiting  circumference,  hav- 
ing only  a  limited  thickness  or  rarified  gas  to  take 
light  from,  would  give  by  comparison  only  negligibly 
faint  effects. 

According  to  this  view  the  solar  phenomena,  sun- 
spots,  for  example,  need  not  be  regarded  as  superficial, 
but  may  lie  at  any  point  between  the  outer  limiting 
sphere  and  the  inner  sphere  to  which  the  diminishing 
spiral  of  refraction  of  the  line  of  sight  at  length  be- 
comes tangent.  If  this  is  so,  a  sun-spot  which  we  see 
near  the  limb  may  really  be  somewhere  on  the  oppo- 
site side  of  the  sun  from  the  earth.  This  hypothesis 
has  curious  consequences  if  we  consider  the  apparent 
rotation  of  the  sun  as  measured  by  observing  sun- 
spots.  For  the  supposed  inner  sphere,  on  which  the 
spot  by  hypothesis  really  lies,  must  go  at  a  different 
rate  of  rotation  from  the  apparent  rate  of  the  sun. 
Suppose  the  sun's  equator  in  the  plane  of  the  ecliptic, 

230 


WHAT  IS  THE  SUN? 

and  that  a  certain  equatorial  spot  actually  lay  at  the 
limb  of  the  inner  sphere,  but  appeared  at  the  limb 
of  the  boundary  sphere.  After  a  synodic  day's  ro- 
tation, the  light  pursuing  the  same  actual  path  within 
the  sun  as  before  would  come  out,  it  is  true,  at  a  point 
just  as  far  advanced  in  angle  on  the  boundary  sphere 
as  the  point  it  started  from  was  on  the  inner  sphere ; 
but  coming  out  nearly  tangent  to  the  outer  sphere,  as 
before,  it  would  not  be  directed  towards  the  earth  at 
all.  The  path  of  light  directed  towards  the  earth 
through  a  sun-spot,  advanced  apparently  one  synodic 
day's  march  on  the  boundary  sphere,  would  pursue  an 
entirely  differently  shaped  spiral  within  the  sun,  and 
would  cut  our  hypothetical  sun-spot  sphere  on  its 
equator,  to  be  sure,  but  not  at  the  same  angular  de- 
parture from  the  first  position  as  would  be  indicated 
by  the  appearances.  But  when  we  reach  the  center 
of  the  sun's  disk  the  line  of  sight  is  straight.  Hence, 
the  total  period  of  rotation  of  the  supposed  inner  sun- 
spot  sphere  must  equal  that  of  the  apparent  rotation 
outside ;  for  every  time  the  apparent  sun-spot  reaches 
the  center  of  the  disk  the  real  one  is  directly  behind  it. 
Accordingly,  the  motion  of  the  supposed  inner  sun- 
spot  sphere  must  be  non-uniform,  which  seems  ab- 
surd. The  sun-spot  must,  therefore,  be  really,  as  well 
as  apparently,  superficial.  An  interesting  result  of 
Schmidt's  hypothesis  appears,  also,  if  we  consider 
spectroscopic  line-of-sight  determinations  of  solar 
rotation.  For  the  motion  in  the  line  of  sight  depends 
on  how  far  down  in  the  sun  we  consider  the  light  as 
17  231 


THE   SUN 

arising,  so  that  it  would  seem  that  all  spectral  lines 
should  be  widened,  when  viewed  near  the  limb,  unless 
the  material  which  gives  rise  to  them  is  situated  close 
to  the  apparent  level  of  the  limb. 

Schmidt's  views  have  obtained  considerable  ac- 
ceptance, but  not  from  observers  of  solar  phenomena. 
The  late  Professor  Keeler  said:1  " According  to  this 
theory,  the  sharpness  of  the  sun's  limb  and  the  enor- 
mous change  of  brightness  at  that  place  are  not 
caused  by  corresponding  abrupt  changes  in  the  con- 
stitution, density,  or  light-radiating  power  of  the 
solar  matter,  but  are  the  result  of  refraction  in  a  non- 
homogeneous  medium.  ...  In  other  words,  the 
photosphere  is  an  optical  and  not  a  material  sur- 
face. .  .  .  Various  assumptions  as  to  the  mass, 
temperature,  etc.,  are  here  necessary,  which  it  is  gen- 
erally impossible  to  verify,  but  Dr.  Knopf  has 
shown  .  .  .  that  the  conditions  in  the  case  of  the 
sun  are  well  within  the  bounds  of  probability.  .  .  . 
But,  however  difficult  it  may  be  for  present  theories 
to  account  for  the  tenuity  of  the  solar  atmosphere, 
immediately  above  the  photosphere,  and  however 
readily  the  same  fact  may  be  accounted  for  by  the 
theory  of  Schmidt,  it  is  certain  that  the  observer 
who  has  studied  the  structure  of  the  sun's  surface, 
and  particularly  the  aspect  of  the  spots  and  other 
markings  as  they  approach  the  limb,  must  feel  con- 
vinced that  these  forms  actually  occur  at  practically 

1  Astrophysical  Journal,  vol.  i,  p.  178,  1895. 
232 


WHAT   IS   THE   SUN? 


the  same  level,  that  is,  that  the  photosphere  is  an 
actual  and  not  an  optical  surface. " 


D's 


JULIUS'S  VIEWS 

Professor  W.  H.  Julius  of  Utrecht  has  proposed  a 
group  of  solar  theories  composed  of  ingenious  appli- 
cations of  the  principles  of  anomalous  dispersion.  It 
has  been  abun- 
dantly shown  by 
laboratory  ex- 
periments that 
the  dispersion  of 
light  by  the  {MO 


vapors  of  metals  a999 
is  subject  to  dis- 
continuities in 
the  regions  of 
spectrum  im-  0.9% 
mediately  adja- 
cent to  their  lines 
of  strong  emis- 
sion and  absorp- 
tion. Fig.  57  shows  the  anomalous  two-branched  dis- 
persion curve  of  sodium  vapor  in  the  neighborhood  of 
the  D  lines,  according  to  researches  of  R.  W.  Wood. 
For  comparison,  the  normal  dispersion  of  rock-salt  in 
the  same  region  is  also  given.  The  enormous  vari- 
ations of  dispersion  of  the  light  on  the  edges  of  the  D 
lines  by  sodium  vapor  would  cause  the  production  of 
dark  spectral  lines  under  certain  circumstances,  not 

233 


4.547 

1 

\ 

\6 

V 

\s 

^ 

—  -.     - 

—  '              „. 

, 

—  —  —  . 

^x 

\ 

^4 

\ 

X 

\ 

3 

\ 

;v 

1 

5OO    56        57        58        59        6O        61         62       6 

FIG.  57. — NORMAL  AND  ANOMALOUS 
DISPERSION. 


THE  SUN 

by  true  absorption,  but  by  anomalous  dispersion. 
Julius  has  applied  this  to  the  explanation  of  many  of 
the  solar  phenomena,  and  the  reader  interested  should 
consult  his  numerous  papers  and  also  the  critical 
articles  of  Hartmann,  Anderson,  and  others.  See 
The  Astrophysical  Journal,  Asironomische  Nachrichten, 
et  cetera. 

We  may  briefly  state  two  or  three  of  Julius's  ex- 
planations here,  and  first  concerning  the  chromo- 
sphere and  prominences.  These  objects  have  bright 
line  spectra,  and  appear  to  protrude  beyond  the  limb 
of  the  sun.  Eruptive  prominences  often  appear  to 
shoot  out  as  rapidly  as  100  miles  a  second!  But  to 
Julius  they  are  not  seen  by  their  own  brilliance  out- 
side the  sun's  limb,  nor  do  they  rise  with  such  veloci- 
ties at  all.  The  line  of  sight  to  the  apparent  summit 
of  a  prominence  is  really,  he  thinks,  a  greatly  curved 
line  by  virtue  of  the  anomalous  dispersion  caused  by 
the  non-homogeneous  density  of  a  mass  of  non-lum- 
inous gas  existing  there;  and  the  true  source  of  the 
principal  light  is  in  the  photosphere.  A  slight  re- 
arrangement of  the  density  alters  greatly  the  path  of 
the  rays,  and  causes  the  impression  of  displacement 
of  the  prominence  at  enormous  speeds.  Adjacent 
wave  lengths  of  the  photospheric  light  do  not  reach 
the  observer,  because  not  anomalously  refracted. 
The  wave  lengths  of  prominence  spectra,  if  unaffected 
by  other  causes,  would  generally  be  slightly  greater 
than  the  wave  lengths  of  the  true  absorption  lines  of 
the  gases  concerned,  because  the  density  must,  on 

234 


WHAT   IS   THE   SUN? 

the  whole,  diminish  outside  the  photosphere.  But 
Julius  regards  irregular  density  gradients  in  the  oppo- 
site direction  as  of  common  occurrence,  so  that  short 
wave  lengths  will  frequently  occur.  The  displace- 
ments of  wave  lengths  themselves  will,  he  thinks,  be 
almost  imperceptible.  Whatever  may  be  our  opinion  of 
Julius's  explanation  of  the  high  prominences,  we  must, 
I  think,  admit  a  considerable  probability  that  anomal- 
ous dispersion  might  produce  many  of  the  phenomena 
of  the  chromosphere.  But,  on  the  other  hand,  if  the 
chromospheric  gases  are  self-luminous,  the  anomalous 
dispersion  effects  may  be  almost  entirely  masked. 

Fraunhofer  lines  Julius  regards  as  "  absorption 
lines  enveloped  in  dispersion  bands,"  the  latter 
caused  by  a  honeycomb  of  irregular  density  gradients 
in  the  photosphere,  and  showing  themselves  chiefly 
as  the  "  wings"  which  occur  with  many  lines.  Rever- 
sals of  chromospheric  lines  he  regards  as  evidence  of 
local  condensations  of  gas,  in  which  density  gradients 
in  both  directions  occur,  thus  bringing  both  longer 
and  shorter  wave-length  dispersion  bands  to  the  eye. 

Even  sun-spots  he  attributes  to  refraction,  but  not 
anomalous  refraction,  at  least  as  regards  their  major 
phenomena.  He  imagines  local  strong  condensations 
or  rarefactions  in  the  photosphere,  and  shows  how 
these  might  produce  regions  of  diminished  radiation, 
on  account  of  the  re-distribution  of  rays,  and  the 
return  of  some  to  the  sun.1 

1  S:H>  further  "Regular  Consequences  of  Irregular  Refraction  in 
Ihe  Sun,"  by  W.  H.  Julius,  Proc.  Roy.  Acad.  of  Amsterdam,  Meet- 

235 


THE   SUN 

Astronomers  generally  admit  that  in  the  sun  there 
may  be  conditions  which  favor  the  production  of 
phenomena  of  anomalous  dispersion,  especially  in  the 
chromosphere.  With  few  exceptions,  however,  they 
believe  anomalous  effects  negligible,  and  the  observed 
facts  to  be  more  simply  and  satisfactorily  accounted 
for  on.  the  basis  of  ordinary  views  of  selective  emis- 
sion and  absorption,  such  as  have  been  given  in  pre- 
ceding chapters.  The  test  between  the  two  methods 
of  explanation  often  involves  the  precise  measure- 
ment of  wave-lengths,  and  such  criteria  have  not  thus 
far  been  applied  with  such  rigor  as  to  exclude  entirely 
the  explanations  advanced  by  Julius.  It  is  not  im- 
possible that  writers  on  solar  phenomena  ten  years 
hence  ^will  devote  much  space  to  the  discussion  of 
anomalous  dispersion. 

THE  AUTHOR'S  VIEWS 

We  must  leave  the  reader  to  supplement  by  his 
reading  of  the  original  papers  these  inadequate  sum- 
maries of  the  views  of  various  investigators,  and  we 
will  pass  on  to  the  solar  theory  which  seems  to  the 
writer  most  probable.  In  its  most  general  aspect 
this  is  similar  to  the  views  stated  by  Secchi  in  1877 
for  Newcomb's  " Popular  Astronomy."  Also  an 
important  paper  by  Schuster  entitled  "Radiation 

ing  of  Sept,  25,  1909;  ''On  the  Origin  of  the  Chromospheric 
Light,"  Meeting  of  same  Academy,  Nov.  27,  1909.  "Anomalous 
Refraction  Phenomena  Investigated  with  the  Spectroheliograph," 
by  W.  H.  Julius,  Astrophysical  Journal,  Dec.,  1908,  et  cetera. 

236 


WHAT  IS  THE  SUN? 

Through  a  Foggy  Atmosphere"  1  has  some  things  in 
common  with  it.  Still  more  in  touch  is  the  paper  of 
Schwartzchild  already  mentioned.2 

It  will  be  assumed :  A.  The  sun,  excepting  perhaps 
in  sun-spots,  is  wholly  gaseous  or  vaporous.  Except 
in  sun-spots  the  photosphere  is  too  hot  to  contain 
solids  or  liquids.  B.  The  density  of  the  gases  rapidly 
diminishes,  and  their  temperature  rapidly  falls  from 
within  outwards  across  the  apparent  boundary  of  the 
sun. 

The  view  that  the  sun's  photosphere  is  too  hot  to 
contain  other  than  gaseous  constituents  has  been 
strongly  combated  by  J.  F.  Hermann  Schulz,3  who 
even  argues  that  the  sun  is  mainly  liquid.  He  sets 
the  average  temperature  of  the  photosphere  at  5,400° 
C.  (5,673°  Abs.)  Although  admitting  that  the  late 
H.  Moissan  placed  the  temperature  of  his  electric 
furnace  at  3,500°  C.,  and  stated  that  all  known  ele- 
ments volatilize  at  that  temperature,  Schulz  argues 
that  the  temperature  of  the  electric  furnace  is  to  be 
set  higher,  even  probably  as  high  as  the  sun's  temper- 
ature, and  the  volatilization  is  not  to  be  regarded  as 
complete  in  the  furnace.  His  argument  is  that  the 
enormous  energy  of  the  electric  current  used  (see 
table  below)  had  no  adequate  escape  by  conduction 
or  radiation,  and  must  have  raised  the  temperature 


1  Astrophysical  Journal,  vol.  xxi,  pp.  1-22,  1905. 
2 "  Ueber  das  Gleichgewicht  der  Sonnenatmosphare,"  Gottingen 
Nachr.,  Math-phys.     Kl.     1906,  pp.  1-13. 

3  Astrophysical  Journal,  vol.  xxix,  pp.  33-39,  1909. 

237 


THE  SUN 


of  the  furnace  till  checked  by  the  melting  and  evapo- 
ration of  the  limestone  of  which  it  was  constructed. 

He  continues:  "Now  Moissan  has  shown  that,  even 
at  the  enormous  temperature  attained  in  his  electric 
furnace,  we  have  not  yet  reached  the  point  at  which 
all  terrestrial  elements  are  truly  boiling.  In  this  re- 
spect the  following  table  is  very  instructive,  which 
Moissan  gave  in  Comptes  Rendus  of  February  19, 
1906  (142,  430). " 

TABLE  XV. — Moissan's  experiments  on  the  vaporization  of  metals  of 
the  iron  family 


Metal 

Weights 
Grams 

Time 
Minutes 

Amperes 

Volts 

Metal 
Distilled 
Grams 

Nickel  

150 
200 

5 
9 

500 
500 

110 
100 

56 
200 

Iron  

150 

825 
800 

5 
10 
20 

500 
1000 
1000 

110 
55 
110 

14 
150 
400 

Manganese  

150 
150 

3 
5 

500 
500 

110 
110 

38 
80 

Chromium  

150 

5 

500 

110 

38 

Molybdenum  

150' 
150 

10 

20 

700 
700 

110 
110 

0 
56 

Tungsten  

150 

20 

800 

110 

.   25 

Uranium  

150 
150 

200 

5 
5 
9 

500 
700 
900 

110 
110 
110 

0 
15 
200 

"Moissan  further  adds  the  following  remarks: 
'  Molybdenum.  The  150  grams  were  not  fused  by  a 
current  of  500  amperes  and  110  volts.  After  applying 

238 


WHAT  IS  THE  SUN? 

700  amperes  and  110  volts  for  seven  minutes,  the 
metal  was  fused  but  nothing  evaporated.  After 
twenty  minutes  56  grams  were  distilled.  Tungsten. 
After  applying  500  amperes  and  110  volts  for  five 
minutes,  the  metal  was  not  yet  fused.  After  apply- 
ing 800  amperes  and  110  volts  for  twenty  minutes, 
boiling  commenced,  but  only  25  grams  distilled. ' 

"Another  highly  interesting  paper  of  Moissan  is: 
'Sur  la  distillation  des  corps  simples. '  l  Here  we  find 
the  following  statement: 

" '  Gold  commences  to  evaporate  in  vacuo  at  1,070°. 
It  boils  in  vacuo  at  1,800°,  and  should  boil  at  760  mm. 
pressure  at  2,530°, '  thus  showing  how  much  depends 
upon  the  pressure  under  which  boiling  takes  place. 
Now  all  Moissan's  experiments,  tabulated  above, 
are  made  at  ordinary  atmospheric  pressure,  and  we 
are  entirely  at  loss  to  say  how  much  the  evaporation 
of  the  various  metals  would  have  been  retarded  under 
increased  pressure,  such  as  we  might  expect  at  the 
very  base  of  the  solar  atmosphere,  close  to  the  liquid 
nucleus. 

"Moissan  tried,  also,  the  metalloid  titanium  in  his 
electric  furnace.  Five  hundred  grams  were  heated 
by  a  current  of  500  amperes  and  110  volts;  after  four 
minutes  vapor  appeared,  but  after  five  minutes  the 
stuff  was  fused  only  on  the  surface,  and  carbide  of 
titanium  had  formed.  Then  300  grams  were  treated 
with  1,000  amperes  and  55  volts  for  seven  minutes; 
110  grams  were  distilled;  the  stuff  itself,  however, 

1  Annexes  de  chemie  et  de  physique,  (8)  8,  145-181,  1906. ' 
239 


THE   SUN 

had  been  only  viscous,  the  surface  had  not  become 
horizontal.  In  his  book  'Der  electrische  Of  en/  p. 
238,  he  says  that  even  with  a  current  of  2,200  am- 
peres and  60  volts,  the  stuff  in  the  crucible  is  not  com- 
pletely fused." 

The  behavior  of  titanium  is  not  unparalleled.  Some 
substances  go  over  from  solids  to  gases  without  melt- 
ing at  all.  In  the  second  experiment  nearly  half  of 
the  material  was  distilled,  although  the  melting  was 
not  complete.  No  substances  are  cited  which  failed 
to  become  largely  gaseous  under  a  few  minutes  heat- 
ing at  atmospheric  pressure  with  the  electric  oven. 
True,  an  increase  of  pressure  to  five  or  ten  atmos- 
pheres, which  may  prevail  in  layers  we  can  see  in  the 
sun,  would  certainly  have  hindered  the  evaporation. 
But  if  the  electric  oven  is  above  3,500°  C.,  even  4,000° 
C.,  it  is  still  far  beneath  the  photospheric  tempera- 
ture. For  if  the  solar  constant  is  1.95  calories,  as  will 
be  shown  in  Chapter  VII,  the  photosphere  cannot  be 
at  a  lower  temperature  than  5,860°  Absolute  Centi- 
grade, and  may  be  much  higher  if  its  intrinsic  radiat- 
ing capacity  is  considerably  less  than  that  of  the  per- 
fect radiator.  Indeed,  it  seems  most  probable  that 
the  photospheric  temperature  should  be  set  not  lower 
than  6,500°  Absolute.  At  such  a  temperature,  pre- 
vailing not  minutes  but  milleniums,  one  can  most 
easily  believe  all  elements  are  entirely  gaseous. 

As  for  the  sun  being  mainly  liquid,  as  argued  by 
Schulz,  the  sun's  low  specific  gravity  has  led  even 
those  who  prefer  to  believe  in  a  cloudy  photosphere 

£40 


WHAT  IS   THE   SUN? 

to  regard  the  interior  as  almost  wholly  gaseous.  We 
return  to  our  discussion. 

It  is  required  to  explain :  1.  Why  the  sun  presents 
a  sharp  boundary.  2.  Why  the  enormous  radiation 
of  the  photosphere  does  not  so  far  cool  its  surface  as  to 
precipitate  clouds.  3.  Why  a  more  or  less  definite 
structure  appears  on  the  sun.  4.  Why  the  spectrum 
of  the  sun  is  mainly  continuous.  5.  Why,  towards  the 
limb,  there  is  a  gradual  decrease  in  brightness,  and  an 
alteration  in  spectral  distribution.  6.  Why  the  solar 
spectrum  has  dark  lines. 

Besides  these  principal  requirements,  there  are  a 
thousand  details  of  fact  not  necessary  here  to  re- 
hearse, which  must  not  be  hopelessly  inconsistent 
with  any  satisfactory  solar  theory.  Finally  there  are 
the  great  problems  of  the  periodicity  of  sun-spots, 
faculse,  et  cetera,  the  variations  of  solar  rotation  with 
latitude,  and  the  supply  of  the  sun's  energy. 

(1)  Why  the  Sun  Presents  a  Sharp  Boundary. 

In  Lord  Rayleigh's  celebrated  mathematical  in- 
vestigations of  the  light  of  the  sky  he  has  shown  that, 
whether  proceeding  on  the  hypothesis  of  the  elastic 
solid  theory  of  light,  or  on  the  electromagnetic  theory, 
the  extinguishing  effect  on  a  beam  of  light  of  the 
molecules  of  a  gas,  or  of  a  collection  of  particles  which 
are  small  compared  with  the  wave  length  of  light, 

327rV  ~  I)2 
may  be  expressed  by  the  relation :  k  =  •         ^ ; 

oJN 
in  which  k  is  the  coefficient  of  extinction,  p  is'  the 

24:1 


THE  SUN 

index  of  refraction,  and  N  is  the  number  of  particles, 
or  molecules,  per  cubic  centimeter.  Schuster  has 
proved  the  relation  to  be  independent  of  theory  if  ^ 
is  approximately  unity.  This  is  true  for  all  gases. 
He  has  applied  this  quantitative  theory  of  extinction 
to  the  atmosphere.1  For  N  he  uses  Rutherford  and 
Geiger's  value,  2.72  x  1019  molecules  per  cubic  cen- 
timeter. If  h  is  the  height  of  the  homogeneous  at- 
mosphere, that  is,  the  height  to  which  the  atmosphere 
would  extend  if  entirely  at  standard  temperature  and 
pressure,  then  e~kh  is  the  fraction  of  light  which 
would  reach  the  observer  if  none  were  lost  in  any 
other  way  than  by  molecular  scattering.  From  these 
data  Schuster  calculates  the  extinction  above  sea- 
level,  and  above  1,800  meters  elevation,  and  compares 

TABLE    XVI. — Difference    between    observed    and    computed    values 
of  atmospheric  transmission 


Washington  Observed 

Mt.  Wilson  Observed 

Wave 

Computed 

Computed 

Length 

-"Clear" 

-"Clear" 

Mean 

Clear 

Mean 

Clear 

0/*.4 

0.55 

0.72 

—0.01 

0.73 

0.76 

0.00 

0.5 

0.70 

0.84 

+  0.03 

0.85 

0.89 

0.00 

0.6 

0.76 

0.87 

0.07 

0.89 

0.92 

0.03 

0.7 

0.84 

0.90 

0.06 

0.94 

0.96 

0.01 

0.8 

0.87 

0.94 

0.04 

0.96 

0.99 

—0.01 

1.0 

0.90 

0.96 

0.03 

0.97 

0.99 

0.00 

the  computed  values  with  the  transmission  observed 
on  days  of  mean  and  maximum  "transparency  at 
Washington  and  Mount  Wilson,  respectively,  by 
Smithsonian  observers. 


1  Nature,  vol.  Ixxxi,  p. 
242 


1909. 


WHAT   IS   THE   SUN? 

Schuster  concludes  that  on  a  clear  day  on  Mount 
Wilson  molecular  scattering  practically  accounts  for 
the  atmospheric  extinction.  Even  at  Washington 
he  thinks  the  major  part  of  the  losses  in  the  atmos- 
phere may  be  thus  accounted  for;  although  on  the 
average  day  something  must  be  attributed  to  re- 
flection and  absorption  of  grosser  dust  particles. 

Professor  Natanson  has  treated  the  matter  from 
the  standpoint  of  the  electron  theory.  He  differs  in 
some  respects  from  Ray  lei  gh  and  Schuster,  although 
deriving  a  practically  similar  formula  for  scattering, 
for  he  introduces  not  the  number  of  molecules  but  the 
number  of  electrons  per  cubic  centimeter.  He  also 
has  compared  theory  with  the  observations  of  Smith- 
sonian observers  at  Washington  and  Mount  Wilson, 
and  finds  an  approximate  agreement.  He  does  not 
state  the  conclusion  in  so  many  words,  but  his  results 
indicate  that  the  extinction  of  light  above  Mount  Wil- 
son on  the  best  days  may  reasonably  be  accounted  for 
by  scattering  of  the  gas  itself  without  consideration  of 
dust  particles. 

All  this  has  apparently  a  very  important  bearing 
on  our  views  of  the  sun.  The  temperature  of  the 
layers  from  which  we  get  the  most  light,  as  already 
stated,  seems  to  be  certainly  in  excess  of  6,000°  Ab- 
solute Centigrade.  There  are  no  substances,  so  far  as 
known,  which  can  exist  except  as  vapors  in  these  con- 
ditions. Hence,  it  seems  reasonable  to  suppose  that 
the  sun  contains  no  solids  or  liquids,  unless  perhaps 
in  sun-spots,  and  that  its  substance,  as  we  see  it;  and 

243 


THE  SUN 

within  the  layers  we  see,  is  altogether  gaseous.  But 
if  this  is  so,  how,  it  will  be  asked,  can  the  sun  present 
a  sharp  boundary? 

According  to  the  theory  of  Schmidt,  which  has  been 
alluded  to,  this  is  caused  by  the  effect  of  refraction. 
But  if  Rayleigh  and  Schuster  and  Natanson  are  right 
in  attributing  a  substantial  light  scattering  effect  to 
gases,  Schmidt's  theory  needs  hardly  to  be  invoked, 
nor,  indeed,  can  it  really  be  of  much  application.  For 
if,  as  computed  by  Schuster,  the  quantity  of  gas  in 
the  vertical  column  of  atmosphere  above  Mount  Wil- 
son is  sufficient  to  scatter  from  the  direct  beam  of 
yellow  sunlight  six  per  cent  of  its  light,  a  column  con- 
taining seventy-five  times  as  much  will  suffice  to  scat- 
ter ninety-nine  per  cent. 

Several  observers  have  found  that  the  pressure  in 
the  reversing  layer  for  iron  is  about  five  atmospheres. 
Assuming  the  average  absolute  temperature  of  the 
photosphere  to  be  6,500°,  and  that  of  the  air  250°,  the 
quantity  of  gas  per  cubic  centimeter  in  the  reversing 
layer  would  be  about  |-  as  great  as  in  air  at  atmos- 
pheric pressure.  As  the  homogeneous  atmosphere 
above  Mount  Wilson  is  less  than  ten  miles  high, 
seventy-five  times  the  quantity  of  gas  above  Mount 
Wilson  would  be  found  probably  within  4,500  miles 
of  the  top  of  the  sun's  reversing  layer.  This  estimate 
assumes  the  line  of  sight  radial  within  the  sun,  and 
regards  five  atmospheres  as  the  average  pressure.  If, 
as  Evershed  maintains,  the  pressure  of  the  reversing 
layer  is  only  of  the  order  of  one  atmosphere,  still  we 

244 


WHAT  IS  THE  SUN? 

must  admit  that  the  pressure  increases  rapidly  with 
the  depth,  so  that  still  the  estimate  seems  to  be  ample. 

Hence,  it  seems  probable  that  gaseous  scattering 
alone  prevents  us  from  seeing  towards  the  center  of 
the  sun,  when  looking  directly  at  the  middle  of  the 
solar  disk,  to  more  than  5,000  miles  below  the  re- 
versing layer. 

At  the  limb  of  the  sun,  the  direct  line  of  sight  to  a 
position  at  the  same  distance  radially  below  the  re- 
versing layer  would  traverse  fully  60,000  miles  of  gas. 
Accordingly,  to  obtain  our  column  containing  the 
requisite  quantity  of  gas  for  practical  extinction  of 
yellow  light,  at  the  limb  we  should  penetrate  a 
layer  which,  measured  along  the  radius,  would  be 
very  much  thinner  than  that  required  at  the  center 
of  the  disk.  For,  even  to  a  radial  depth  of  only  500 
miles,  the  direct  line  of  sight  is  almost  20,000  miles. 

These  considerations  seem  to  point  to  a  reasonable 
explanation  of  the  sharp  boundary  of  the  sun.  For 
at  the  edge  of  the  disk,  owing  to  the  oblique  line  of 
sight,  gaseous  scattering  will  probably  extinguish 
almost  all  yellow  light  starting  from  more  than  500 
miles  below  the  chromosphere,  while  an  even  less 
thickness  suffices  for  blue  or  violet  light.  It  is  plain 
that  an  indistinctness  of  outline  corresponding  to  a 
layer  of  this  depth  would  not  be  readily  recognized 
on  the  solar  image,  since  it  corresponds  to  only  about 
one  second  of  arc.  Furthermore,  the  direct  line  of 
sight  takes  in  not  only  the  nearer,  but  the  further 
solar  hemisphere  as  well.  A  still  thinner  stratum 

245 


THE  SUN 

than  500  miles  would,  therefore,  suffice  to  contribute 
all  the  light  that  can  be  contributed  to  the  beam  di- 
rectly along  the  line  of  sight.  We  therefore  con- 
clude that  within  a  small  part  of  a  second  of  arc  below 
the  reversing  layer  the  sun  would  appear  as  a  solid 
body,  even  though  entirely  gaseous.1 

(2)  Why  Is  there  No  Cloudy  Photosphere? 

But  it  is  said  by  Young  and  many  others  that  a 
cloudy  photosphere  must  certainly  exist  as  the  result 
of  the  juxtaposition  of  the  hot  gases  of  the  sun  with 
the  cold  of  space.  Without  falling  back  on  the  strong 
reply  that  the  apparent  temperature  of  the  so-called 
photosphere  exceeds  6,000°  Absolute  Centigrade,  and 
that  no  known  substances  can  exist  except  as  vapors 
at  that  temperature,  it  may  be  asked  whether  the  ab- 
sence of  a  cloud  immediately  above  the  smoke-stack 
of  a  locomotive  in  winter  does  not  show  that  such  a 
juxtaposition  of  hot  gases  and  cold  surroundings  with- 
out forming  a  cloud  is  entirely  possible.  There  is  no 
cloud  formed  immediately  above  the  smoke-stack 
because  the  steam  there  is  superheated  above  the 
boiling  point.  It  may  be  urged  that  a  little  time  is, 
of  course,  required  to  form  the  cloud,  and  that,  owing 
to  the  rapid  motion  of  the  steam,  it  is  carried  a  little 
above  the  smoke-stack  during  this  interval.  But 
this  is  really  admitting  that  while  the  steam  remains 
superheated  it  will  not  form  a  cloud,  so  that  all  that  is 

1  For  practical  purposes  of  seeing,  it  is  not  the  depth  of  the  layer 
which  scatters  ninty-nine  per  cent.,  but  a  much  less  fraction  that  is 
in  question. 

246 


WHAT  IS  THE  SUN? 

necessary  to  prevent  a  cloud  is  to  supply  heat  to  the 
steam  as  fast  as  heat  escapes  from  it,  and  thus  to  keep 
it  superheated. 

Such  a  state  of  affairs  seems  to  exist  in  the  sun. 
Heating  is  communicated  from  the  interior  to  the  sur- 
face layers  fast  enough  to  maintain  the  latter  above 
6,000°,  notwithstanding  their  radiation  to  space,  and 
at  this  temperature  no  cloud  forms.  The  convey- 
ance of  heat  from  within  is  probably  almost  wholly  by 
repeated  radiation,  rather  than  by  vertical  convection 
currents.1 

(3)  What,  then,  Is  the  Cause  of  the  So-called ' '  Rice-grain 
Structure"  on  the  Sun,  if  there  Are  No  Clouds? 

It  is  not  to  be  supposed  that  the  communication  of 
heat  from  within  outwards  is  perfectly  uniform  at  all 
parts,  for,  as  evidenced  by  the  sun-spots,  the  prom- 
inences, and  the  corona,  there  are  marked  defects  of 
homogeneity  in  the  sun.  Hence,  it  may  readily  be 
supposed  that  some  regions  of  the  gas  are  a  little  hot- 
ter than  others,  and  that  these  differences  of  tempera- 
ture will  give  rise  to  differences  of  brightness.  By  the 
radiation  laws,  the  increase  of  brightness  is  far  more 
rapid  than  the  corresponding  increase  of  temperature. 

Professor  J.  Scheiner  published,  in  1895,  a  theory 
of  the  solar  granulation  which  seems  very  reasonable; 
and  which,  if  we  consider  the  effects  produced  to  be 
merely  regions  of  local  cooling  without  actual  con- 
densations, would  suit  the  theory  of  the  altogether- 

1  This  is  the  view  of  Schwartzchild  and  also  of  See. 
18  247 


THE  SUN 

gaseous  sun  as  well  as  it  does  the  theory  of  the  cloudy 
photosphere. 

Professor  Scheiner  says  (quoting  from  a  translation 
in  the  Astrophysical  Journal) :  "  According  to  the  the- 
ory of  Helmholtz,  air  waves  are  produced  when  two 
layers  of  air,  differing  in  temperature  (i.  e.  in  density), 
glide  past  each  other,  just  as  waves  are  produced  by 
the  gliding  of  air  over  water.  If  the  lower  layer  is 
nearly  saturated  with  aqueous  vapor,  condensations 
will  take  place  in  the  wave  crests  on  account  of  the 
diminution  of  pressure.  Under  these  circumstances 
the  elevations  or  wave  crests  appear  as  clouds,  the 
depressions  or  troughs  as  transparent  interspaces, 
and  thus  a  more  or  less  regular  series  of  cirrus  clouds 
is  produced.  If  the  impulses  resulting  in  wave  forma- 
tion act  in  two  different  directions  the  waves  cross, 
and  we  have  the  cloud  effect  known  as  a  mackerel  sky. 
The  great  similarity  in  appearance  between  the  solar 
photosphere  and  terrestrial  cirrus  has  long  been  rec- 
ognized, and  there  is  no  doubt  that  the  necessary  con- 
ditions for  the  application  of  Helmholtz 's  theory  to 
the  solar  atmosphere — the  existence  of  layers  of  dif- 
ferent temperature,  the  over-saturated  state  of  con- 
densable gases  (in  the  photosphere),  and  variously 
directed  currents  in  the  different  layers — are  found  in 
the  sun.  I  therefore  regard  the  bright  grains  of  the 
photosphere  as  wave  crests,  rendered  visible  by  con- 
densation, or  at  least  an  increase  of  condensation,  of 
two  crossing  series  of  waves." 

We  may  adopt  Schemer's  view  in  the  present  dis- 
248 


WHAT  IS   THE  SUN? 

cussion,  only  not  admitting  actual  condensation. 
Hence,  his  bright  grains  would  be  our  dark  ones,  be- 
cause the  cooler  regions  would  radiate  least.  The 
reader  will  see  that  this  amendment  to  Schemer's  in- 
terpretation is  rendered  at  least  plausible  by  the  fact 
that  spectroheliograms  show  bright  and  dark  hydro- 
gen flocculi,  and  of  course  no  such  a  thing  as  a  con- 
densed hydrogen  cloud  can  be  thought  of  at  solar  tem- 
peratures. 

(4)  Why  Is  the  Sun's  Spectrum  Mainly  Continuous? 
Gases  are  noted  for  giving  only  line  spectra,  while 
the  solar  spectrum  is,  on  the  contrary,  chiefly  a  con- 
tinuous spectrum  crossed  by  absorption  lines.  In 
reply  to  this  objection  it  may  be  said  that  gases  under 
pressure  give  more  and  more  continuous  spectrum 
along  with  the  bright  lines,  even  in  layers  of  small 
thickness,  like  those  operated  on  in  the  laboratory. 
(See  Plate  XVIII.)  Think,  then,  if  layers  many  miles 
thick,  and  under  pressures  of  at  least  several  atmos- 
pheres, may  not  give  a  fully  continuous  spectrum. 

(5}  Why  Does  the  Limb  Fall  Off  in  Brightness  and 

Grow  Redder? 

As  stated  above,  the  light  received  from  near  the 
edge  of  the  solar  disk  comes,  on  the  whole,  from  more 
superficial  layers  than  that  received  from  the  center 
of  the  disk;  because  at  the  edge  we  look  obliquely, 
and  hence  by  a  longer  path,  into  the  sun,  and  the  scat- 
tering of  the  molecules  cuts  off  the  view  before  the 
deeper  layers  seen  at  the  center  are  reached.  At  -the 

249 


THE  SUN 


edge,  the  layers  which  are  emitting  light  to  us,  being 
more  superficial,  and  hence  cooler,  will  in  consequence 
give  less  intense  light  than  those  at  the  center.  • 

Referring  to  Tables  7  and  8,  Chapter  III,  it  is  pos- 
sible to  compute,  either  by  Stefan's  law  or  by  Wien's 
law,  the  change  in  effective  temperature  required  to 
account  for  the  decrease  of  brightness  towards  the 
sun's  limb.  As  shown  in  Table  8,  the  two  methods  of 
computation  are  in  close  accord.  Extending  the 
result  somewhat,  we  have  the  following  differences  of 
temperature,  assuming  the  central  disk  temperature 
6,400°  Absolute  Centigrade.  These  may  be  compared 
with  the  corresponding  differences  of  elevation  of 

the  lowest  observable  layer,  assuming  a  depth  of  — — 

100 

radius,  or  7,000  kilometers,  as  the  limit  of  visibility 
at  the  center  of  the  disk. 


Fraction  of  radius 
from  center  of 
sun's  disk  

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

Decrease  of  temper- 
ature   

0° 

20° 

45° 

80° 

115° 

160° 

Increase  of  elevation 
of  farthest  visible 
layer  

Okm. 

66km. 

140km. 

315km. 

545km. 

930km. 

The  small  temperature  gradient  of  the  order  of 
1°  C.  per  kilometer  of  change  of  level1  required  for 
this  line  of  explanation  seems  no  greater  than  we 
should  expect  to  exist  in  the  sun's  outer  layers. 

As  scattering  is  greater  for  violet  than  for  red  rays, 
the  violet  rays  will  come,  on  the  average,  from  more 

1  Mean  radiating  level,  not  lowest  visible  level. 
250 


WHAT  IS  THE  SUN? 

superficial  layers  than  the  red,  both  at  the  center  and 
edge.  Accordingly,  the  diameter  of  the  sun  should  be 
greater  if  measured  in  violet  light  than  if  measured  in 
red,  so  far  as  this  consideration  goes.  But  the  differ- 
ence of  diameter  due  to  this  cause  is  probably  too 
little  to  be  measured.  It  is  obscured  by  " boiling"  of 
the  sun's  image,  diffraction,  and  scattered  light  in  the 
earth's  atmosphere,  any  one  of  which  alone  probably 
produces  a  greater  effect  at  the  limbs  than  that  we 
are  considering.  According  to  Planck's  formula,  the 
change  of  intensity  of  radiation  accompanying  change 
of  temperature  of  the  radiating  source  is  greater  pro- 
portionally for  short  wave  lengths  than  for  longer 
ones.  Hence,  it  follows  that  the  violet  should  be 
weaker  with  respect  to  the  red  at  the  limb  than  at  the 
center  of  the  sun.  This  is  in  accord  with  observation. 
Whether  this  effect  would  be  augmented  or  dimin- 
ished in  consequence  of  the  fact  that  the  effective 
radiating  layer  for  violet  radiation  is  nearer  the  sur- 
face than  that  for  red  at  both  center  and  edge,  de- 
pends on  the  relative  change  of  temperature  due  to 
this  shifting  of  depth  at  the  two  regions.  It  seems 
impossible  as  yet  to  determine  how  this  would  be. 

(6)  Why  Has  the  Solar  Spectrum  Dark  Lines  ? 

All  the  Fraunhofer  lines  would  really  be  bright  if 
seen  against  a  dark  background.1  They  are  dark  only 
relatively  to  the  brighter  continuous  spectrum.  In 

1  Different  persons  estimate  their  brightness  as  from  one-fifth  to 
one-tenth  that  of  the  continuous  spectrum  background. 

851 


THE  SUN 

these  lines  the  selective  absorption  of  radiation  is  very 
powerful,  and  cuts  off  all  transmission  within  a  short 
distance,  so  that,  as  compared  with  the  continuous 
spectrum,  they  are  emitted  very  near  the  surface  of 
the  sun.  This  superficial  layer  in  which  they  arise  is 
cooler  than  that  which  lies  behind,  hence  its  emission 
is  less  intense,  and  hence  the  comparative  darkness  of 
the  Fraunhofer  lines.  As  between  the  center  and  the 
limb  of  the  sun  we  should  expect  little  change  in  the 
absolute  brightness  of  the  Fraunhofer  lines,  because, 
owing  to  the  powerful  selective  absorption  within 
them,  they  are  very  markedly  superficial  phenomena 
both  at  center  and  limb.  Thus,  but  little  change  in 
the  effective  depth  and  temperature  from  which  they 
are  emitted  occurs,  no  matter  from  what  angle  the 
surface  of  the  sun  is  viewed.  It  is  not  so  with  the 
process  of  weakening  by  scattering,  which  requires 
great  thickness  of  gas;  and  hence,  as  we  have  seen, 
the  continuous  spectrum  is  brighter  at  the  center  of 
the  sun  than  at  the  limb.  Consequently  the  contrast 
or  " intensity"  of  Fraunhofer  lines  falls  off  towards 
the  limb,  because  they  change  little,  while  the  back- 
ground against  which  they  are  seen  falls  off  in  bright- 
ness. 

Why  Are    Not  all    Chemical   Elements    Impartially 
Represented  by  the  Intensities  of  Their  Solar  Lines  f 

It  is  not  to  be  inferred  from  what  has  been  said 
under  (6)  that  there  is  no  thickness  to  the  "reversing 
layer,"  or  no  change  of  its  effective  thickness  from 


WHAT   IS   THE  SUN? 

the  center  to  the  edge  of  the  sun,  but  only  that, 
relatively  to  the  effective  thickness  of  the  layer 
which  furnishes  the  continuous  spectrum  at  the 
center  of  the  sun's  disk,  the  reversing  layer  for 
any  one  element  is  thin.  Hence,  we  may  dis- 
tinguish between  high  level  and  low  level  spectrum 
lines.  It  would  be  expected  a  priori  that  elements 
(a)  of  high  atomic  weight,  (b)  of  high  vaporizing 
temperature  would  be  found  at  lower  levels,  and 
(c)  that  of  the  spectrum  lines  of  a  single  element  the 
longer  wave  lengths  would,  so  far  as  depending  on  the 
relations  of  temperature  and  emission,  represent 
higher  levels.1  It  might  perhaps  be  expected  that  the 
reversing  layer  for  a  heavy  element  could  lie  wholly 
below  that  of  a  light  one.  For  very  low  lying  ele- 
ments it  might  conceivably  occur,  through  scattering, 
that  their  entire  spectra  would  disappear  at  the  edge  of 
the  sun,  although  appearing  at  the  center.  In  general, 
low  lying  elements  would  give  weak  solar  spectra, 
because  the  temperature  of  the  emission  of  their  lines 
would  more  nearly  approach  the  temperature  of  the 
emission  of  the  continuous  spectrum  background. 

Referring  to  Chapter  III,  the  reader  will  recall  the 
marked  connection  between  atomic  weight  and  in- 
tensity of  solar  spectra.  On  the  whole,  the  elements 
of  less  atomic  weight  give  the  strongest  solar  spectra. 
The  platinum  group,  of  very  high  atomic  weight,  on 
the  other  hand,  is  only  partly  represented  in  the  solar 

1  The  effect  of  scattering  would  tend  in  the  other  direction,  how- 
ever. 

'253 


THE  SUN 

spectrum.  Rowland  and  Tatnall  say/  speaking  of 
these  elements:  "The  heavier  lines  have  been  exam- 
ined as  to  the  probability  of  their  occurrence  in  the 
solar  spectrum,  and  investigation  has  confirmed  the 
existence  of  rhodium  and  palladium  in  the  sun.  Ruth- 
enium is  doubtful"  (afterwards  confirmed)  "and  it  is 
most  probable  that  there  are  no  solar  lines  of  appre- 
ciable intensity  belonging  to  platinum  or  osmium  in 
this  region  of  the  spectrum"  (\3,000  to  4,000).  "The 
most  intense  lines  of  the  arc  spectra  of  rhodium  and 
palladium  correspbnd  to  extremely  weak  solar  lines." 
This  failure  of  solar  lines  is  not  for  lack  of  strong  lines  in 
the  arc,  for  Rowland  and  Tatnall  give  many  platinum 
arc  lines  of  intensities  5  to  15,  to  which  there  are  cer- 
tainly no  corresponding  solar  lines  above  intensity  00. 

The  comparison  of  intensities  and  atomic  weights 
given  in  Chapter  III  has  some  glaring  discrepancies. 
Carbon  is  found  near  lanthanum,  although  its  atomic 
weight  is  but  12.  It  is  now  believed  that  solar  lines 
attributed  by  Rowland  to  carbon  are  really  due  to 
carbon  compounds  of  considerable  molecular  weight, 
notably  to  cyanogen.  Glucinum  and  potassium  fall 
in  strange  company.  But  they  have  only  one  or  two 
lines  each  identified  by  Rowland,  and  these  may  lead 
us  into  error.  Indeed,  Kayser  and  Runge  question 
the  existence  of  potassium  lines  in  the  photospheric 
spectrum. 

In  the  flash  spectrum  at  eclipses  we  have  another 
indication  of  differences  of  level.  There  again,  as 

1  Astrophysical  'Journal,  vol.  ii,  p.  184,  1895. 
254 


WHAT   IS   THE   SUN? 

shown  by  Evershed,  Lockyer,  Jewell,  Mitchell,  and 
others  from  measurements  of  the  lengths  of  flash 
spectrum  arcs,  the  order  of  level  agrees  on  the  whole 
with  that  which  we  have  just  considered.1  Again,  in 
Adams'  work  on  the  solar  rotation,  if  we  grant  (as  we 
must  when  we  recall  the  relative  rotational  velocities 
observed  through  red  hydrogen  (Ha),  calcium 
(\4,227),  and  iron  lines)  that  lower  levels  correspond 
to  slower  velocities,  then  we  find  (CN2)  and  lan- 
thanum falling  in  at  lower  levels  than  iron  and  titan- 
ium, just  as  they  appear  to  do  from  considerations  of 
Rowland's  intensities. 

The  absence  of  lines  of  helium,  the  halogens  and 
other  negative  elements  in  the  photospheric  spectrum 
is  probably  due  to  the  extinguishing  effect  which  the 
metals  appear  to  produce  on  the  lines  of  such  elements 
when  the  metallic  and  other  gases  are  mixed.  Thus, 
according  to  E.  Wiedemann,  nitrogen  and  hydrogen 
lines  first  begin  to  appear  in  a  vacuum  tube  showing 
mercury  lines  when  the  concentration  of  these  gases 
is  thirty  per  cent.  Also,  common  salt  in  a  flame  shows 
the  spectrum  of  sodium  alone,  not  of  chlorine. 

What  Causes  the  Differences  of  Character  and  Wave 
Length  for  Fraunhofer  Lines  between  the  Center 
and  Edge  of  the  Sun  f 

The  reader  will  recall  that,  after  allowing  for  the 
rotation  of  the  sun  and  for  an  apparent  general  rise 

1  The  extremely  high  level  of  calcium  H  and  K  lines  is  an 
anomaly  not  well  understood. 

255 


THE  SUN 

of  the  brighter  material  toward  the  solar  surface, 
Adams  1  confirmed  Halm's  and  Buisson  and  Fabry's 
results  that  there  is  a  general  displacement  toward  the 
red  of  the  centers  of  most  solar  lines  as  seen  near  the 
limbs  and  compared  with  the  center.  This  displace- 
ment is  inappreciable  for  the  more  prominent  lines 
of  hydrogen,  calcium,  sodium,  and  magnesium,  and 
small  for  the  other  lines  of  these  elements.  Also  for 
elements  of  high  atomic  weight  the  shifts  are  very 
small.  Iron  and  nickel  lines  show  larger  shifts  than 
those  of  titanium,  vanadium,  and  scandium.  En- 
hanced lines  as  a  class  show  larger  shifts  than  arc 
lines  do.  Lines  strengthened  at  the  limb  show  small 
shifts.  The  displacements  are  greater  for  long  wave 
lengths  than  for  short.  The  character  of  lines  at  the 
limb  is  also  altered.  Some  strong  lines  of  the  elements 
hydrogen,  sodium,  calcium,  silicon,  magnesium, 
aluminum,  iron,  chromium,  titanium,  and  manganese 
lose  partially  or  wholly  the  winged  appearance  which 
they  have  at  the  center.  Many  lines  of  all  kinds  of 
elements  are  slightly  widened.  The  enhanced  lines 
and  lines  of  elements  of  high  atomic  weight  are  gen- 
erally much  weakened. 

The  weakening  of  high  temperature  and  low  lying 
element  lines  may  be  attributed  to  scattering.  At 
the  center  of  the  sun  we  look  straight  down  upon  the 
lower  reversing  layers  and  get  their  rays  under  more 
favorable  angles  of  scattering  than  at  the  limb,  where 
they,  in  order  to  contribute  to  the  line  of  sight,  must 

1  Contributions  of  the  Mount  Wilson  Solar  Observatory,  No.  43. 

356 


WHAT   IS   THE   SUN? 

scatter  by  one  or  more  reflections  through  a  right 
angle,  or  nearly  so.  Hence  the  continuous  spectrum 
at  the  limb  encroaches  upon  their  lines,  diluting  them 
with  stray  light.  Furthermore,  what  is  at  least 
equally  important,  the  sun's  continuous  spectrum  is 
weaker  at  the  limb,  for  reasons  already  considered, 
and  would  give  the  lines  less  contrast,  even  without 
the  effect  we  have  just  considered.  This  weakening- 
of  the  continuous  spectrum  at  the  limb  contributes 
powerfully  to  reduce  the  visibility  of  the  wings  of 
lines,  also,  because  the  wings  are  seen  against  a  back* 
ground  which,  towards  the  limbs,  approaches  more 
and  more  their  own  strength  of  emission.  The  widen- 
ing of  lines  seems  possibly  a  promiscuous  Dopplei 
effect  due  to  their  being  contributed  to  by  different 
levels  rotating  at  different  velocities. 

Adams  explains  the  superior  displacements  of  en- 
hanced lines  by  suggesting  that  at  the  center  of  the 
sun,  the  higher  temperature  gases  are  rising,  the 
cooler  ones  falling,  giving  for  the  spectrum  lines  in 
general  a  rising  effect;  because  most  of  the  light 
comes  from  the  brighter  emitting  matter  which  is 
rising.  But  for  the  lines  which  are  high  temperature, 
or  enhanced,  lines  a  maximum  rate  of  rise  (greater 
than  that  of  average  lines)  is  observed,  because  the 
descending  cooler  vapors  do  not  emit  or  absorb  en- 
hanced lines,  so  that  for  these  lines  there  is  a  displace- 
ment of  central  spectra  towards  the  violet  which  ap- 
pears as  an  increased  displacement  of  edge  spectra 
towards  the  red.  Of  course,  at  the  limb  these mo- 

257 


THE   SUN 

tions  of  rise  and  fall  are  at  right  angles  to  the  line  of 
sight,  and,  therefore,  produce  no  Doppler  effects. 

Having  thus  cleared  the  ground  of  Doppler  effects 
of  rotation  and  rise,  Adams  attributes  the  remaining 
displacements  to  pressure  depending  on  level.  High 
level  lines  are  not  displaced  because  emitted  under 
slight  pressure  both  at  the  edge  and  center.  Low 
level  lines  are  not  displaced  because  they  arise  only 
from  thin  strata  at  the  very  bottom  of  the  layer 
which  is  visible  to  us,  and  which  must  be  at  nearly 
equal,  though  high  pressures  at  both  center  and 
limbs.  Scattering  does  not  permit  us  to  see  much 
beyond  the  outer  boundaries  of  such  strata  at  center, 
and  at  the  limb  we  see  them  only  faintly,  and  after 
the  rays  have  been  one  or  more  times  reflected,  hence 
such  spectrum  lines  are  weak  at  the  limb.  Lines  of 
intermediate  levels  are  under  higher  effective  pres- 
sures at  the  limb  than  at  the  center,  according  to 
Halm's  view,  as  adopted  by  Adams,  because  any  line 
of  sight  drawn  just  inside  the  limb  has  a  longer  rela- 
tive path  in  the  lower  layers  it  cuts  than  a  line  of 
sight  drawn  near  the  center  of  the  disk  has  in  the 
corresponding  layers.  Hence,  lower  layers  contrib- 
ute proportionately  more  to  the  spectra  of  inter- 
mediately lying  elements  at  the  limb  than  at  the 
center. 

The  writer  must  confess  that  he  feels  a  little  hesi- 
tancy about  adopting  this  last  argument,  because  he 
thinks  that  it  would  be  necessary  to  consider  for  these 
layers  quite  as  much  the  rays  scattered  into  the  beam 

258 


WHAT  IS  THE  SUN? 

from  all  sides  as  to  consider  merely  the  line  of  sight. 
But  until  the  proportions  contributed  to  a  beam  by 
scattering  from  different  distances,  and  at  different 
angles,  and  the  rate  of  change  of  density  along  the 
sun's  radius,  are  better  known  than  now,  it  seems  idle 
to  press  this  objection. 

Julius  has  explained  the  displacements  of  lines 
towards  the  red  as  a  simple  consequence  of  anomalous 
dispersion.  But  Adams  shows  that  the  lines  apt  to  be 
most  powerfully  affected  by  anomalous  dispersion 
show  no  shifts  at  all,  and  that  a  comparison  of  all  the 
known  data  as  to  the  strength  of  anomalous  disper- 
sion for  the  several  lines  with  their  observed  shifts  at 
the  limb  yields  nothing  to  recommend  this  explana- 
tion of  Julius. 

Why  Do  the  Prominences  and  the  Chromosphere  Give 

Bright  Line  Spectra  f 

According  to  the  line  of  explanation  we  are  pursu- 
ing, the  gases  of  these  appendages  of  the  sun  are  in  a 
condition  of  extremely  low  pressure  and  density,  and 
do  not  contain  sufficiently  many  molecules  contrib- 
uting radiation  to  the  line  of  sight  to  emit  a  strong 
continuous  spectrum.  But  for  the  spectrum  lines  of 
powerful  selective  emission,  their  radiation  is  suffi- 
ciently considerable  to  reveal  their  form's. 

What    of    the    Characteristic  Forms    and   Occasional 
Immense  Velocities  of  the  Prominences  ? 
Although  not  shared  by  all,  there  has  always  been  a 

hesitancy  among  many  of  those  who  regard  prom- 

259 


THE  SUN 

inences  as  real  protruding  masses  of  bright  gas,  and 
not,  after  Julius,  as  mirage  effects,  to  trust  their  ob- 
servations, both  direct  and  spectroscopic,  that  these 
gaseous  masses  are  bodily  projected  at  such  rates  as  a 
hundred  miles  a  second.  It  is  hard  to  imagine  on 
purely  mechanical  grounds  how  such  velocities  could 
arise.  In  spectroscopic  determinations  the  motion 
observed  in  prominences  is  apparently  tangential  to 
the  sun.  W.  A.  Michelson  of  Russia  has  suggested 
that  in  this  case  we  may  really  have  moderate  motion 

across    the    line    of    sight, 

as  illustrated  in  the  accom- 
FlG  58  panying  Fig.  58.    Let  as  be 

a  line  of  sight,  and  let  a 

mass  of  gas  whose  front  is  AA,  rise  to  positions  BB 
and  CC.  Then  the  source  of  light  moves  effectively 
from  a  to  b  to  c,  giving  an  apparently  enormous 
motion  in  the  line  of  sight,  which  is  really  a  much 
smaller  motion  across  the  line  of  sight.  Whatever 
may  be  thought  of  this  explanation,  it,  of  course,  has 
reference  only  to  apparent  enormous  tangential  ve- 
locities. 

As  for  apparent  enormous  radial  motions,  we  all 
frequently  see  wisps  of  cirrus  clouds  stretch  across  the 
sky  in  a  twinkling,  as  it  were.  This  does  not,  of 
course,  indicate  motion  of  translation  from  one  end 
of  the  wisp  to  the  other,  but  rather  the  rise  of  a 
trough  of  cooling,  which  causes  a  precipitation  of 
cloud  almost  simultaneously  along  its  whole  length. 
Young  and  others  state  that  detached  prominences 

260 


WHAT   IS   THE  SUN? 

sometimes  form  without  preliminary  attachment  to 
the  sun's  chromosphere.  Perhaps  eruptive  promi- 
nences are  formed  somewhat  as  cirrus  clouds  are,  by 
the  rise  from  the  sun  of  some  disturbance  fitted  to 
arouse  emission  almost  simultaneously  within  large 
masses  of  previously  non-luminous  hydrogen  and 
calcium  gases,  which  lie  where  a  prominence  is  about 
to  appear.  Perhaps  an  electrical  excitation  would  be 
most  reasonable.  The  apparent  tremendous  velocity 
of  the  outbreak  could  then  be  explained  by  reference 
to  the  accompanying  dia- 
gram, Fig.  59.  Let  the  line 
of  sight  be  in  the  line  of 
the  arrow  I,  AB  the  photo- 
sphere, and  ab  the  trough  which  suddenly  arouses 
emission  beginning  at  a  and  proceeding  almost  im- 
mediately to  6.  The  lower  end,  a,  is  lost  in  the  glare  of 
the  photosphere,  and  the  prominence  appears  to  rise 
from  c  to  b  in  the  very  brief  time  needed  to  extend  to 
6  the  influence  of  the  trough.  If  the  line  of  sight 
had  been  in  the  direction  of  the  arrow  II  the  promi- 
nence would  have  appeared  detached.  The  writer 
does  not  venture  to  recommend  this  suggestion  very 
strongly. 

Professor  E.  Pringsheim1  has  suggested  an  explan- 
ation of  these  enormous  observed  prominence  veloci- 
ties that  appears  very  reasonable.  He  refers  to  ex- 
periments of  J.  Stark,  who  has  found  Doppler  dis- 
placements of  the  order  of  magnitude  which  occur  in 

1  E.  Pringsheim,  "  Physik  der  Sonne,"  Leipzig,  1910,  pp.  225-228. 

261 


THE  SUN 

prominences,  when  observing  the  so-called  "  canal 
rays,"  which  are  curious  electrical  discharges  ob- 
tained through  rare  gases  by  special  contrivance. 
This  indicates  that  the  positively  charged  atoms, 
which  give  the  light,  may  travel  with  velocities  like 
those  which  appear  in  the  prominences,  when  forced 
by  ordinary  differences  of  electrical  potential  in  very 
rare  gases.  Pringsheim  then  draws  attention  to  the 
fact  that  the  comet  of  1843  passed  at  perihelion  within 
3'  or  4'  of  the  photosphere  (TO*  the  solar  diameter) 
without  being  affected  by  the  resistance  of  the 
material  encountered.  This  proves  the  existence  of 
a  sufficient,  indeed,  of  an  extraordinary  degree  of 
vacuity  there.  It  is  not  known  that  sufficient 
variations  of  electrical  potential  exist  in  the  sun's 
neighborhood,  but  those  which  exist  in  the  earth's 
atmosphere  are  abundantly  sufficient  to  drive  elec- 
trons with  prominence-like  velocities  in  vacuum,  ac- 
cording to  Pringsheim's  computations.  The  exist- 
ence of  similar  potential  gradients  near  the  sun 
seems  not  improbable. 

Julius's  explanation  of  prominences  through  anom- 
alous dispersion  we  have  already  noted,  but  it 
requires  us  to  admit  the  propagation  of  disturbances 
to  the  apparent  tops  of  the  prominences,  and  to 
believe  that  the  gases  at  such  enormous  heights  are 
dense  enough  to  produce  appreciable  anomalous  re- 
fraction. 


262 


WHAT   IS  THE  SUN? 

What  Is  the  Corona? 

Comets  pass  through  the  corona  without  sensible 
retardation.  Hence,  its  matter,  whether  it  be  purely 
gaseous  or  partly  meteoric  dust,  must  be  very  rare. 
Since  the  corona  gives  the  Fraunhofer  spectrum  in  its 
outer  part,  it  must  contain  reflected  photospheric 
light.  Its  substance  in  its  inner  parts  may  well  be  hot 
enough,  by  virtue  of  proximity  to  the  sun,  to  give 
light  of  incandescence.  Its  form  suggests  the  auroral 
streamers,  and  inclines  one  to  think  that,  as  the  au- 
roral light  is  of  electrical  luminescence,  so  may  a  part 
of  the  coronal  light  be.  As  the  aurora  gives  bright 
lines  in  its  spectrum,  so,  also,  does  the  corona.  The 
proportions  of  the  mixture  of  these  three  varieties 
in  coronal  radiation  is  unknown,  but,  according  to 
bolometric  work,  the  mixture  gives  almost  the  same 
spectral  distribution  in  the  inner  corona  as  photo- 
spheric  radiation.  This  suggests  that  light  of  lumi- 
nescence and  of  reflection  together  predominate  over 
incandescence. 

On  the  other  hand,  the  results  obtained  by  repre- 
sentatives of  the  Lick  Observatory  at  several  eclipses 
have  led  Campbell,  Perrine,  and  Lewis  to  express  the 
opinion  very  definitely  that  the  inner  corona  shines 
mainly  by  ordinary  incandescence,  due  to  the  heating 
of  its  particles  on  account  of  the  absorption  by  them 
of  photospheric  radiation.  Neglecting  the  bolometric 
results,  this  conclusion  would  be  perfectly  reasonable 
and  it  may  yet  prove  that  there  is  some  error  in  these 
19  263 


THE  SUN 

latter  results  which  may  explain  the  discrepancy. 
Yet  the  conditions  of  the  bolometric  work  at  Flint 
Island  were  so  satisfactory,  and  the  observations  so 
concordant,  that  this  seems  rather  improbable.  Per- 
haps some  new  line  of  explanation  may  suit  all 
parties. 

The  electrical  explanation  of  the  coronal  form  and 
brightness  is  receiving  much  attention.  Professor 
Pringsheim  devotes  much  space  to  it  in  his  new  work, 
"Physik  der  Sonne."  In  a  recent  article,  however, 
Professor  R.  W.  Wood1  has  sought  to  explain  both 
the  polarization  and  light  emission  of  the  corona  as 
the  effects  of  the  passage  of  the  powerful  sun-rays 
through  comparatively  cool  metallic  vapors,  thereby 
exciting  fluorescent  light  in  them.  By  laboratory 
experiments  with  light  so  excited  in  vapors  of  sodium, 
potassium,  and  iodine,  he  finds  the  percentage  of  po- 
larization similar  to  that  in  the  corona.  He  states 
that  the  spectrum  of  mixed  vapors  would  be  contin- 
uous, at  least  for  low  dispersion.  The  fluorescent 
spectrum  is,  in  fact,  made  up  of  thousands  of  fine 
lines  arranged  in  groups  and  bands,  and  gives  no  re- 
semblance to  the  bright  line  spectra  of  the  same  ele- 
ments. These  lines  lie  so  closely  packed  as  probably 
to  escape  detection  with  low  dispersion  spectroscopes. 
Any  color  of  fluorescence  may  occur,  according  to  the 
kind  of  vapors  mixed,  and  their  proportions.  Wood 
thinks  it  quite  possible  that  the  coronal  green  line  is 

1  Astrophysical  Journal,  vol.  xxviii,  p.  75,  1908. 
264 


WHAT  IS  THE  SUN? 

not  a  bright  line  of  some  unknown  substance,  but 
rather  a  yet  unrecognized  fluorescent  line  from  some 
well-known  element. 

Schaberle  has  long  maintained  a  mechanical  erup- 
tion theory  of  the  coronal  form.1  He  traces  back  the 
probable  courses  of  the  streamers,  and  locates  them 
in  centers  of  eruption  on  the  sun's  disk.  His  views 
agree  well  with  the  hypothesis  that  the  coronal  bright- 
ness is  mainly  of  incandescence. 

The  cause  of  the  change  of  form  of  the  corona  with 
the  sun-spot  cycle  is  unknown. 

Importance  of  Temperature. 

It  will  be  noted  that  in  the  solar  hypotheses  we  are 
recommending  the  temperature  plays  a  most  promi- 
nent part.  First  of  all,  the  existence  of  a  cloudy  pho- 
tosphere is  denied  because  the  temperature  of  the 
photosphere  is  shown  probably  to  reach  6,500°,  for  it 
is  highly  improbable  that  solids  or  liquids  can  exist  in 
such  conditions.  Secondly,  the  presence  of  the  so- 
called  "  granulations  "  is  regarded  as  evidence  of  dif- 
ferences of  temperature  in  the  radiating  gas — dif- 
ferences which  would  naturally  be  expected  in  an  im- 
mense globe  of  gas  giving  off  tremendous  amounts  of 
radiation  from  its  surface,  and  known  to  present  irreg- 
ularities of  rotation  and  cyclonic  motions  in  addition. 
Thirdly,  the  darkening  towards  the  limb  is  regarded 
primarily  as  a  temperature  effect,  secondarily  due  to 

1  See  Lick  Observatory  Contributions,  vol.  iv,  1893. 
265 


THE  SUN 

scattering.  Owing  to  scattering,  the  effective  radi- 
ating layer  must  necessarily  be  nearer  the  surface, 
and  hence  cooler,  at  the  limb  than  at  the  center  of  the 
disk.  We  say  it  must  be  nearer  the  surface:  For, 
travelling  obliquely,  a  ray  must  become  extinguished 
by  scattering  in  the  gas  at  the  limb,  before  it  reaches 
the  same  radial  depth  that  it  does  if  travelling  radi- 
ally at  the  center.  Fourthly,  the  darkening  at  the 
limb  would  naturally  be  greater  for  violet  than  for 
red  rays,  firstly,  because  with  all  incandescent  bodies 
a  fall  of  temperature  causes  more  decrease  of  radiation 
for  short  rays  than  for  long;  and,  secondly,  because 
molecular  scattering  is  greater  for  violet  rays  than  for 
red,  and  hence  at  the  sun's  edge  the  effective  radiating 
layer  for  the  violet  will  be  more  near  the  surface  than 
will  that  for  the  red.  Fifthly,  the  Fraunhofer  lines  are 
regarded,  not  as  dark,  but  as  very  bright,  intrinsi- 
cally. They  only  appear  dark  because,  owing  to  pow- 
erful selective  absorption  of  the  gases  which  give  rise 
to  them,  they  cut  off  completely  the  light  from  be- 
hind, and  the  observer  sees  only  a  relatively  thin  and 
superficial  layer  of  the  sun,  when  viewing  it  by  the 
light  of  the  Fraunhofer  lines.  The  reversing  layer  is 
hence  colder,  and  its  radiation  less  intense  than  that 
of  the  continuous  spectrum  background  which  comes 
from  deeper  layers  of  the  sun.  Sixthly,  the  contrast 
of  the  Fraunhofer  lines  with  the  background  of  spec- 
trum decreases  as  our  view  approaches  the  edge  of 
the  sun's  disk,  because  the  Fraunhofer  line  region  is  so 
thin  and  superficial  that  its  temperature  is  nearly  the 

266 


WHAT  IS  THE  SUN? 

same  at  the  edge  as  at  the  center;  whereas  for  the 
continuous  spectrum  background,  the  effective  radi- 
ating layer  rapidly  approaches  the  sun's  surface  as  we 
look  nearer  the  limb,  and  hence  its  radiation  de- 
creases, owing  to  the  fall  of  temperature. 

What  of  Sun-spots  f 

We  now  trace  the  importance  of  temperature  in  the 
explanation  of  sun-spot  phenomena.  In  accordance 
with  the  Mount  Wilson  observations  of  Hale,  Eller- 
man,  and  St.  John,  we  may  regard  sun-spots  as  vor- 
tices, and,  as  indicated  by  the  spectrum  work  of  Ever- 
shed,  we  must  conclude  that  in  the  Fraunhofer  line 
region  the  motion  along  the  spiral  is  from  within  out- 
ward. We  may  imagine  that  these  vortices  are  sim- 
ilar in  form  to  water-spouts  seen  at  sea,  with  the 
trumpet-shaped  part  at  the  top,  and  the  whirl  carry- 
ing matter  from  below  outward.  In  such  circum- 
stances there  would  be  a  great  cooling  of  the  gases, 
owing  to  their  rapid  expansion  as  they  approach  the 
limb.  This  cooling  (as  appears  from  the  discovery  of 
lines  due  to  the  copious  presence  of  calcium  and  mag- 
nesium hydrides,  and  also  of  titanium  oxide  in  sun- 
spot  spectra)  carries  the  temperature  down  to  per- 
haps 3,500°,  which  is  low  enough  for  the  formation  of 
liquids,  and  perhaps  some  solids. 

These  dissimilar  substances,  by  their  friction  (per- 
haps even  by  their  very  formation)  we  suppose  may 
give  rise  to  charges  of  electricity,  which,  being  carried 
round  rapidly  in  the  stem  of  the  vortex,  produce  the 

267 


THE  SUN 

effect  of  currents  of  electricity  as  shown  by  Rowland. 
Hence  they  give  rise  to  the  magnetic  field,  which 
Hale  finds  is  a  feature  of  a  sun-spot.  The  top  of  the 
vortex,  we  assume,  corresponds  in  level  nearly  with 
the  upper  Fraunhofer  line  region.  There  the  cooled 
matter  spreads  out  some  distance  in  spirals  which 
grow  in  radius  so  rapidly  as  to  be  almost  radial  to  the 
umbra.  As  there  is  no  longer  further  rise  and  expan- 
sion, at  length  the  matter  becomes  warmed,  by  con- 
tact, to  the  temperature  of  the  surroundings.  The 
stem  of  the  vortex  is  the  umbra  of  a  sun-spot,  the 
spreading  top  is  the  penumbra. 

The  peculiarities  of  the  sun-spot  spectrum  and  the 
causes  of  these  peculiarities  have  been  dealt  with  at 
considerable  length  in  Chapter  V.  We  may  summar- 
ize them  as  the  peculiarities  attending,  (a)  diminished 
temperature  as  compared  with  the  photosphere,  (6) 
the  action  of  strong  magnetic  fields.  The  sun-spot 
spectrum  has  been  shown  by  Hale  and  Adams  to  be 
the  type  of  the  spectrum  of  the  red  stars.  Since  we 
now  know  that,  as  regards  the  characteristics  for 
which  this  comparison  holds,  the  sun-spot  spectrum 
results  from  the  mere  cooling  of  the  photospheric 
material,  this  relation  is  very  significant,  and  indi- 
cates distinctly  one  step  in  the  process  of  stellar  evo- 
lution. We  shall  recur  to  this  in  Chapter  X. 

We  have  noted  particularly  in  Chapter  V  the  re- 
markable behavior  of  the  hydrogen  lines  in  sun-spots. 
They  are  all  weakened,  and  the  shorter  wave-length 
lines  most  weakened,  as  compared  with  the  photo- 

268 


WHAT  IS  THE  SUN? 

spheric  hydrogen  spectrum.  The  lines  of  other  ele- 
ments are  generally  strengthened  in  spots.  This 
anomaly  seems  explainable  as  due  to  the  high  level  of 
hydrogen.  This  gas  is  relatively  unaffected  in  posi- 
tion, and  in  fact,  as  St.  John  observed,  is  sucked  in- 
ward and  downward  rather  than  whirled  outward 
and  upward  by  the  cyclonic  motion  in  sun-spots. 
Hence,  its  temperature  and  (in  consequence)  its  radi- 
ation is  rather  increased  than  lowered  by  the  presence 
of  spots.  The  continuous  spectrum  background 
against  which  we  see  the  hydrogen  lines  is,  however, 
weakened  in  spots,  and  thereby  the  contrast  of  the 
hydrogen  lines  is  diminished.  In  other  words,  they 
are  weakened.  Owing  to  the  lower  temperature,  the 
energy  spectrum,  that  is,  the  continuous  spectrum 
background,  in  sun-spots  as  at  the  sun's  limb,  is 
weaker  in  the  violet  as  compared  with  the  red  than  is 
the  ordinary  solar  spectrum.  Thus,  in  spots,  the  radi- 
ation in  the  violet  hydrogen  lines  approaches  more 
nearly  the  brightness  of  the  spectrum  background 
than  that  in  the  red  lines.  Hence,  the  comparatively 
greater  weakening  of  the  shorter  wave-length  hydro- 
gen sun-spot  lines  follows. 

In  the  center  of  the  sun-spot  vortex  there  is  a  ten- 
dency to  form  a  vacuum.  Into  this  partial  void  is 
sucked  the  superincumbent  matter,  which  is  the  high- 
level  hydrogen  of  the  chromosphere  and  prominences. 
Hence  occurs  the  inwardly  directed  radial  motion  of 
this  gas  shown  by  the  Ha  spectroheliograms  at  Mount 
Wilson.  Between  the  Ha  level  gas,  which  is  going  in- 

269 


THE   SUN 

ward,  and  the  Fraunhofer  line  gases,  which  are  going 
outward,  there  must  exist  a  quiescent  region.  Hence, 
the  lower  level  hydrogen  and  the  H!  and  H2  cal- 
cium spectroheliograms  show  little  or  no  evidence  of 
stream  lines  or  other  phenomena  of  stream  motion. 
The  failure  of  Adams  to  discover  differences  in  the 
pressure  of  the  reversing  layer  over  sun-spots  may 
be  regarded  as  confirmatory  of  the  superficial  char- 
acter of  the  reversing  layer,  and  of  the  absence  of 
either  elevation  or  depression  in  the  general  sun-spot 
level. 

As  for  the  cause  of  the  formation  of  sun-spots,  that 
is  all  conjecture.  They  are  generally  preceded  by 
faculse  and,  according  to  Fox,  by  eruptive  promi- 
nences. Perhaps  the  facufce,  which  on  our  tempera- 
ture hypothesis  we  regard  merely  as  regions  of  su- 
perior temperature,  may  be  formed  first,  owing  to  the 
presence  above  them  of  prominence  or  coronal  mat- 
ter. Such  formations  above  would  impede  radiation, 
and  hence  would  cause  the  regions  below  to  be  over- 
heated. Being  overheated,  they  would  tend  to  ex- 
pand, and  by  expansion  would  cause  the  rise  of  ma- 
terial from  below,  owing  to  reduced  pressure  above. 
In  this  outflow  a  rotation  would  usually  be  set  up, 
just  as  in  the  escape  of  water  from  a  spout,  and  thus 
the  sun-spot  would  be  formed.  Once  formed  its 
vortical  motion  would  tend  to  continue,  and  would 
naturally  remain  for  considerable  time.  Hale  has 
noticed  that  the  vortices  of  most  sun-spots  of  the 
southern  hemisphere  go  in  one  direction  and  those  in 

£70 


WHAT   IS   THE   SUN? 

the  northern  in  the  opposite.  This  indeed  is  what 
would  be  expected  in  consequence  of  the  different 
rates  of  rotation  of  the  sun  at  different  latitudes. 
But  it  would  also  be  expected  that  accidental  local 
circumstances,  irrespective  of  this  general  cause, 
might  sometimes  determine  the  rotation  in  opposite 
senses.  This  also  is  in  line  with  observation. 

As  to  the  general  cause  of  the  periodical  changes  (1) 
of  the  form  of  the  corona,  (2)  of  the  areas  of  faculse 
and  (3)  of  the  sun-spot  numbers,  these  also,  are  things 
as  yet  altogether  uncertain.  We  have  already  noted 
Halm's  theory  of  sun-spot  periodicity  as  a  conse- 
quence of  internal  conditions.  Schuster  and  others 
have  suggested  exterior  influences  as  the  operative 
causes  of  the  periodicity.  For  instance,  the  periodic 
returns  of  swarms'  of  meteorites,  and  the  periodic 
returns  of  certain  planetary  configurations  have  been 
mentioned. 

As  for  the  variable  rates  of  solar  rotation  at  dif- 
ferent latitudes  and  depths,  these  have  been  regarded 
by  Wilsing,  Sampson,  Wilczynski,  and  Moulton  as 
vestiges  of  some  ancient  actions  in  which  the  sun  fig- 
ured with  outside  celestial  bodies. 

What  Supplies  the  Solar  Energy  f 

Lastly  comes  the  greatest  problem  of  all:  What 
maintains  the  solar  temperature  despite  the  sun's 
enormous  losses  by  radiation?  These  losses  stagger 
expression  in  figures.  At  90,000,000  miles  (145,000,- 
000  kilometers)  the  average  radiation  is  about' two 

m 


THE  SUN 

calories  per  square  centimeter  per  minute.  Hence 
the  total  emission  of  the  sun  is  about 

22 
2X4X y  X  (14,500,000,000,000)2  caloriesperminute! 

Ordinary  fires  of  coal  are  kept  up  by  the  combination 
of  carbon  with  oxygen.  Except  in  sun-spots  no  com- 
binations are  going  on  in  the  sun.  It  is  so  hot  there 
that  most  compounds  would  separate  into  their  ele- 
ments, instead  of  elements  uniting  with  the  evolution 
of  heat.  If  the  sun  had  no  continuous  supply  of  heat, 
but,  like  a  piece  of  metal  lying  on  the  blacksmith's 
anvil,  had  been  cooling  off,  there  would  have  been 
a  marked  decrease  of  the  earth's  temperature  within 
historical  times.  Geologists  show  that  the  earth  has 
not  varied  more  than  a  few  tens  of  degrees  from  the 
present  temperatures  for  probably  50,000,000  years. 
Indeed,  in  that  remote  past  the  earth's  temperature 
appears  to  have  been  a  little  higher  than  it  is  now. 
Assuming  that  the  sun  emitted  its  present  quota  of 
radiation  during  all  that  interval,  the  problem  of  its 
source  of  supply  has  been,  at  least  until  very  recently, 
insoluble.  Since  the  discovery  of  the  breaking  up  of 
radio-active  materials  to  produce  elements  of  lower 
atomic  weight  with  the  evolution  of  heat,  as  for  in- 
stance in  the  production  of  helium  from  radium,  per- 
haps no  such  difficulty  ought  to  be  regarded  as  in- 
superable. It  is  objected  that  radium  and  uranium 
lines  are  not  found  in  the  solar  spectrum.  We  have 
seen,  however,  that  the  lines  of  the  elements  grow 
more  and  more  feeble  as  the  atomic  weight  increases, 


WHAT   IS   THE  SUN? 

and  that  this  seems  to  be  due  to  the  fact  that  the 
heavy  elements  lie  at  low  levels  in  the  sun.  Hence  it 
is  not  surprising  that  uranium  (238.5)  and  radium 
(226.4)  should  not  show  spectrum  lines  even  if  these 
elements  are  present  in  the  sun.  Dr.  G.  F.  Becker, 
however,  thinks  radium  and  uranium  to  be  elements 
which  form  only  at  temperatures  much  below  those 
of  the  sun.  At  all  events,  it  is  more  satisfactory,  if 
possible,  to  account  for  the  solar  heat  by  known 
causes,  rather  than  to  invoke  radio-activity  of  un- 
discovered materials. 

There  are  certain  circumstances  of  geology  which 
may  indicate  a  diminished  radiation  of  the  sun  in  an- 
cient times.  Although  palms  used  to  flourish  in  the 
arctic  zones,  it  does  not  appear  that  the  tropics  were 
then  much  hotter  if  any  than  now.  As  Manson  in- 
sists, this  uniformity  of  climate  from  the  poles  to  the 
equator  seems  hard  to  reconcile  with  the  present 
zonal  distribution  of  temperature,  if  the  sun  were  then 
as  now  the  principal  source  of  heat,  and  its  effects 
then,  as  now,  zonally  distributed.  On  the  other 
hand,  there  is  accumulating  evidence  that  glaciation 
has  occurred  more  than  once  over  great  regions  of  the 
tropics,  and  most  notably  in  the  Permo-Carbonifer- 
ous  period.  In  that  remote  period,  far  antedating 
the  so-called  " glacial"  or  Pleistocene  period  of  com- 
paratively recent  times,  glaciation  prevailed  in  Aus- 
tralia, Southern  Africa,  Hindustan,  and  perhaps  in 
other  tropical  regions.  It  was  no  mere  sporadic 
mountain-top  affair,  but  probably  a  phenomenon  of 

273 


THE  SUN 

more  imposing  extent  than  even  the  glaciation  of  the 
Pleistocene  Period.1  As  will  be  shown  in  the  next 
chapter  it  seems  very  difficult  to  see  how  such  a  sub- 
tropical glaciation  as  this  could  have  come  about  if  at 
that  time  the  sun's  output  was  substantially  as  great 
as  now.  It  does  not  help  us  to  suppose  that  the  poles 
of  the  earth  were  then  shifted  so  that  these  countries 
were  sub-arctic.  The  area  involved  is  so  vast  that  the 
glaciation  would  still  extend  further  from  the  sup- 
posed pole  than  did  that  of  the  Pleistocene  period. 
Besides,  this  would  bring  one  pole  in  the  vicinity  of 
Mexico,  and  the  Permian  deposits  of  Texas  do  not 
justify  the  inference  of  a  polar  climate  there. 

It  seems  worth  considering  if  the  Permian  and  the 
still  earlier  tropical  glaciations  which  geologists  are 

According  to  Chamberlin  and  Salisbury  ("Geology,"  volume  ii, 
pages  636,  634.  Henry  Holt  &  Co.,  1906):  "The  known  Permo- 
Carboniferous  glaciation  of  Australia,  India,  and  Africa  is  found  in 
two  zones,  the  one  north  and  the  other  south  of  the  equator.  In 
neither  zone  have  the  limits  of  glaciation  been  accurately  deter- 
mined, but  in  the  former  it  is  known  to  have  extended  from  latitude 
18°  to  about  35°  and  probably  still  further  north,  while  in  the  latter 
it  is  known  to  have  extended  from  latitude  21°  to  35°.  In  an 
equatorial  zone  about  40°  in  width  glaciation  has  not  been  dis- 
covered. The  glaciation  of  these  various  countries  has  a  range  of 
about  130°  in  longitude.  Glacial  conditions  must  therefore  have 
prevailed  over  an  area,  or  at  least  about  the  borders  of  an  area  many 
times  as  large  as  that  covered  by  ice  in  the  northern  hemisphere 
during  the  Pleistocene  glacial  period."  Speaking  of  the  Australian 
glaciation  they  say:  "It  is  not  to  be  understood  that  the  phenom- 
ena here  described  are  restricted  to  high  altitudes;  rather  they  are 
known  chiefly  at  low  levels,  descending  in  some  places  nearly  to  the 
sea.  The  altitude  of  this  region  is  not  only  low  now,  but  it  was 
probably  low  during  the  glaciation  as  shown  by  the  relation  of  the 
glacial  deposits  to  the  marine  beds." 

274: 


WHAT  IS  THE  SUN? 

now  recognizing  and  also  the  generally  prevailing 
similarity  of  polar  and  equatorial  climates  in  early 
epochs  do  not  all  point  to  one  of  the  following 
hypotheses : 

(A)  Perhaps  the  sun  in  those  early  times  was  not 
so  nearly  exclusively  as  now  the  earth's  source  of  heat, 
and  the  earth  itself  still  retained  so  much  heat  that  its 
life  was  practically  independent  of  the  sun  except  for 
light.  In  a  later  chapter  we  shall  see  that  under 
other  favoring  conditions  by  no  means  all  of  our  pres- 
ent light  supply  is  necessary  to  promote  maximum 
plant  growth,  and  that  the  red  end  of  the  spectrum, 
which  would  suffer  least  reduction  by  a  decrease  in 
the  solar  temperature,  is  highly  efficient  for  plant 
growth.  Perhaps,  then,  the  sun  has  been  gradually 
growing  in  temperature  and  emission,  and  in  the  Per- 
mian times  had  not  then  become  the  practically  ex- 
clusive source  of  heat  to  the  earth's  surface.  We 
may,  then,  briefly  consider  if  Permian  glaciation  was 
perhaps  due,  as  Manson  has  suggested,  to  a  very  mod- 
erate elevation  of  land  areas  within  a  region  of  a  still 
prevailing  low-lying  cloud  mantle,  with  accompany- 
ing snowy  precipitation.  The  great,  and  it  seems  to 
me  insuperable,  difficulty  which  this  hypothesis  en- 
counters is  to  explain  in  any  reasonable  way  how  the 
earth's  temperature  could  be  maintained  for  millions 
of  years  without  depending  so  completely  on  the  sun 
that  the  explanation  of  the  uniformity  of  climates 
fails.  Furthermore  the  aridity  of  climate  indicated 
by  the  great  Permian  deposits  of  salt  and  gypsum 

275 


THE  SUN 

does  not  speak  for  the  existence  of  a  thick  cloud  man- 
tle. These  difficulties  will  be  further  discussed  in  the 
next  chapter. 

(B)  Perhaps  the  sun  in  very  ancient  times  had  not 
yet  altogether  condensed  to  a  pronounced  nucleus, 
but  still  existed  as  a  nebula  of  very  considerable  size, 
so  that  the  earth  was  illuminated  and  warmed  from 
all  directions,  or  (if  no  part  of  the  nebula  inclosed 
the  earth)  at  least  from  nearly  a  hemisphere.1  This 
of  course  would  promote  uniformity  of  temperatures 
from  the  equator  to  the  poles.  If  thus  receiving  ra- 
diation from  a  very  large  solid  angle,  the  intensity  of 
the  radiation  need  have  been  only  very  slight  indeed 
to  maintain  the  earth's  temperature.  Such  radiation 
might  be  furnished  by  a  cloud  of  small  particles  (not 
gases)  comprising  the  nebula.  Even  if  they  gave  no 
considerable  radiation  of  their  own,  they  would  re- 
flect that  of  the  hotter  solar  nucleus.  On  either  hy- 
pothesis (A)  or  (B)  the  radiation  of  the  ancient  sun, 
or  solar  nebula,  to  outside  space  may  have  been  con- 
siderably smaller  than  the  total  of  the  present  solar 
radiation. 

If  either  of  these  views  or  a  combination  of  both  is 
acceptable,  it  relieves  the  problem  of  solar  radiation 
of  much  of  its  difficulty.  We  may  then  suppose  that 
50,000,000  years  ago  the  total  emission  of  solar  radi- 


1  This  suggestion  was  made  by  Chamberlin  about  twelve  years 
ago.  In  Plate  XXVI,  Fig.  1,  is  shown  a  spiral  nebula  on  edge.  The 
bright  region  at  its  center  is  seen  to  extend  out  of  the  plane  of  the 
spiral  so  as  to  fill  a  large  sphere. 

216 


WHAT  IS  THE  SUN? 

ation  was  considerably  less  than  the  present  emis- 
sion, and  that  it  increased  slowly  for  ages,  reaching 
approximately  the  present  output  at  about  the  Pleis- 
tocene period,  which  was  perhaps  not  over  100,000 
years  ago. 

Helmholtz  in  1853  proposed  a  source  of  solar  en- 
ergy supply  which  is  everywhere  recognized  as  cer- 
tainly very  considerable.  He  pointed  out  that  the 
shrinking  together  of  the  sun  converts  potential  en- 
ergy of  position  into. heat,  just  as  the  falling  of  a 
stone  converts  its  potential  energy  of  position  finally 
into  heat.  Several  authors  have  made  computations 
of  the  quantity  of  energy  which  would  be  available 
from  this  source.  Their  results  have  generally  been 
based  on  the  assumption  that  the  sun  was  originally 
a  nebula  filling  a  sphere  whose  diameter  was  the  orbit 
of  Neptune.  It  appears  that  the  condensation  of 
such  a  nebula  having  the  mass  of  the  sun  would 
have  furnished  thus  far  about  25,000,000  times  as 
much  energy  as  the  sun  now  loses  each  year.  (This 
estimate  is  based  on  a  " Solar  Constant"  of  2.0 
calories  per  square  centimeter  per  minute.) 

According  to  Helmholtz's  view,  a  contraction  of 
about  250  feet  per  year  in  the  sun's  diameter  would 
suffice  to  sustain  the  present  solar  radiation.  ^  At  this 
rate  it  would  require  about  10,000  years  to  reduce 
the  apparent  diameter  of  the  sun  by  one  second  of 
arc,  so  that,  so  far  as  telescopic  observation  is  con- 
cerned, the  contraction  theory  is  tenable,  for  a  change 
of  yfr  second  in  the  solar  diameter  is  unrecognizable. 

277 


THE   SUN 

From  calculations  of  Newcomb  the  sun  will  require  to 
have  shrunk  to  one  half  its  present  size  if  it  maintains 
its  present  rate  of  radiation  for  about  7,000,000  years 
longer.  As  shrinking  cannot  go  on  indefinitely,  nor 
can  the  supply  of  heat  from  this  cause  have  been  in- 
finitely great,  we  must,  from  this  point  of  view,  re- 
gard the  duration  of  life  depending  on  the  sun's  rays 
as  having  had  a  beginning  in  the  remote  past,  and  as 
tending  towards  an  end  at  some  remote  time  in  the 
future. 

There  has  been  much  question  in  recent  years 
whether  Helmholtz's  hypothesis  of  the  sun's  energy 
supply  is  adequate  to  account  for  the  duration  of  life 
upon  the  earth  revealed  by  the  geological  record. 
Joly  has  estimated  from  the  volume  and  salt  contents 
of  the  ocean,  compared  with  the  rates  of  discharge 
and  salinity  of  the  rivers,  that  the  earth's  geological 
age  is  about  80,000,000  years.  G.  F.  Becker  has  re- 
cently revised  the  calculations,  with  allowance  for  a 
more  rapid  discharge  of  salt  in  earlier  periods,  and 
finds  about  50,000,000  years.  On  the  other  hand, 
many  geologists  think  the  thickness  of  the  earth's 
deposited  strata  requires  us  to  admit  more  than  100,- 
000,000  years.  The  duration  of  the  sun's  radiation  at 
present  rate  of  output  apparently  cannot  have  been 
supplied  by  shrinking  alone  for  more  than  25,000,000 
years.  But,  as  has  been  said,  it  seems  plausible  that 
the  solar  radiation  was  formerly  less  considerable  than 
now.  If  so,  we  may  lengthen  several  fold  the  dura- 
tion of  the  supply  by  contraction  of  sufficient  solar 

278 


WHAT  IS  THE  SUN? 

radiation  for  purposes  of  supporting  life  on  the  earth, 
leaving  the  question  of  the  earth's  temperature  main- 
tenance under  the  supposed  circumstances  to  be  dis- 
cussed in  the  following  chapter.  On  these  grounds  we 
may  regard  Helmholtz's  contraction  hypothesis  as 
adequate  to  satisfy  the  requirements  of  geology  and 
physics  in  regard  to  the  source  of  the  sun's  energy. 
Whether  or  not  radio-active  processes  are,  or  have 
been,  considerable  sources  of  solar  energy  is  not  yet 
determined. 


20 


CHAPTER  VII 

THE  SUN  AS  THE  EARTH 's  SOURCE  OF  HEAT 

Causes  of  Low  Temperature  at  High  Altitudes. — Measurement  of 
the  Intensity  of  Sun  Rays. — Dependence  of  Solar  Radiation  on 
Air  Mass. — The  Transmission  of  the  Atmosphere. — The  "  Solar 
Constant  of  Radiation.  "—The  Light  of  the  Sky.— The  De- 
pendence of  the  Earth's  Temperature  on  Radiation. — Fluctuation 
of  Solar  Emission. — Geological  Temperatures. 

NEARLY  all  of  the  heat  of  the  earth's  surface  comes 
directly  from  the  sun's  rays.  The  heat  of  coal  and 
wood  and  the  energy  of  water  power  and  wind,  from 
which  heat  may  be  derived,  are  indirectly  the  effect 
of  solar  rays  either  of  present  or  past  times.  Occasion- 
ally a  person  is  met  with  whose  mind  works  so  curi- 
ously as  to  lead  him  to  deny  that  the  sun  is  hot.  Such 
an  one  almost  invariably  calls  attention  to  the  fact 
that  as  we  ascend  a  mountain,  or  are  carried  up  by  a 
balloon,  the  temperature  falls.  Thus,  although  we 
may  be  actually  approaching  the  sun,  the  heating- 
effects  of  the  solar  rays  become  less  obvious.  Of 
course  the  elevation  possible  for  man  to  attain  is  in- 
significant compared  with  the  radius  of  the  earth's 
orbit,  so  that  no  change  of  solar  radiation  ought  to  be 
appreciable  from  the  change  of  distance  to  the  sun 
involved  in  climbing  a  mountain.  But  a  consider- 
able increase  in  the  intensity  of  the  sun's  rays  attends 

280 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

the  mere  ascending  above  the  lower  dusty  part  of  the 
atmosphere.  Hence  there  is  some  excuse  for  surprise 
at  the  decrease  of  temperature  observed  at  high  alti- 
tudes, which  occurs  notwithstanding  the  increase  in 
the  direct  solar  radiation. 

One  secret  of  this  paradox  lies  in  the  fact  that  the 
sun's  rays  heat  only  objects  which  absorb  them. 
Highly  transparent  objects  like  glass,  or  the  air,  de- 
rive little  heat  by  being  shined  upon ;  for  the  rays  pass 
through  them  almost  unchanged.  Absorbing  sub- 
stances like  lamp-black,  on  the  other  hand,  almost 
entirely  destroy  the  rays  and  convert  their  energy  of 
vibration  into  heat.  Upon  the  surface  of  the  earth 
the  air  is  in  contact  with  such  an  absorbing  substance, 
namely  the  ground,  and  is  warmed  by  contact  with  it. 
At  high  altitudes  the  free  air  has  contact  with  no  ab- 
sorbing substance  to  warm  it,  and  as  it  transmits  sun 
rays  with  great  freedom  it  derives  only  a  little  heat 
from  them  directly.  It  contains,  moreover,  ozone, 
carbon  dioxide,  and  water  vapor  which  all  radiate 
freely  long-wave  rays  and  thus  dissipate  to  space  the 
heat  gained.  Consequently  the  high  air  is  cold,  and 
cools  whatever  it  blows  upon.  Its  cooling  action  on 
the  surfaces  of  mountains  is  greater  on  account  of  the 
high  winds  which  prevail. 

Rising  currents  warm  the  upper  air  less  than  they 
would  do  but  for  the  decrease  of  atmospheric  density 
which  occurs  with  increasing  altitudes.  For  the  air 
currents  which  rise  from  the  heated  surface  of  the 
earth  expand  in  rising,  and  by  expansion  are  sorhe- 

281 


•      THE  SUN 

what  cooled.  A  factor  of  considerable  influence  tend- 
ing to  cause  lower  temperatures  on  elevated  inland 
table  lands,  like  the  plateau  of  Thibet,  is  the  compar- 
ative lack  of  water  vapor  in  the  air  above.  The 
water  evaporated  from  the  Indian  Ocean  can  hardly 
reach  the  plateau  of  Thibet  because  in  rising  through 
the  free  air  to  such  a  great  height  it  is  so  much  cooled 
as  to  be  mostly  precipitated.  Water  vapor,  while 
nearly  transparent  to  light,  and  indeed  to  perhaps 
eighty-five  per  cent  of  all  the  rays  which  the  sun 
sends,  is  on  the  other  hand  a  powerful  absorber  of  the 
rays  of  great  wave  length  which  are  emitted  by  a 
comparatively  cool  body  like  the  earth.  Hence  at 
low  altitudes  where  water  vapor  is  plentiful  in  the 
air,  it  is  a  considerable  hindrance  to  the  escape  of 
earth  rays  to  space.  In  the  comparative  lack  of  water 
vapor  at  high  altitudes  of  interior  regions  of  large 
continents,  the  cooling  of  the  ground  by  radiation  to 
space  is  much  more  rapid  than  at  sea  level,  and  hence 
lower  temperatures  prevail.  In  the  case  of  steep  and 
rough  mountains  the  configuration  of  the  ground  is  con- 
ducive to  low  temperatures  because  it  diminishes  the 
radiation  per  unit  area  received  from  the  sun,  while 
increasing  the  area  affected  by  the  cooling  winds. 

We  may  therefore  attribute  the  coolness  of  the  free 
upper  air  to  its  transparency,  its  considerable  radi- 
ating capacity,  and  its  expansion;  the  coolness  of  the 
rugged  mountains  to  their  contours,  and  to  the  con- 
tact of  the  cool  winds;  the  coolness  of  the  elevated 
inland  plateaus  to  the  dryness  of  the  air  above  them ; 

282 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

at  the  same  time  recognizing  that  they  all  three  are 
receiving  more  intense  rays  from  the  sun  than  is  the 
earth's  surface  in  general. 

MEASUREMENT   OF  THE   INTENSITY    OF    SUN-RAYS 

That  which  the  sun  sends  to  the  earth  in  such  abun- 
dance used  to  be  considered  as  three  distinct  things, 
namely:  Actinic  or  chemical  rays;  light  or  visible 
rays;  heat  or  invisible  rays.  These  distinctions  are 
now  known  to  be  misleading,  for  the  rays  which  affect 
modern  photographic  plates  extend  in  the  spectrum 
from  far  beyond  the  farthest  violet  to  far  beyond  the 
farthest  red,  and  the  rays  which  can  produce  heat  in- 
clude all  these,  and  many  more,  still  further  beyond 
the  red.  All  rays  may  be  totally  transformed  to  pro- 
duce heat,  however  they  may  differ  in  their  effects 
upon  the  eye,  or  on  different  chemical  substances. 
All  these  rays  travel  with  equal  velocity  in  free  space, 
and  this  velocity  is  about  300,000  kilometers  (186,- 
000  miles)  per  second.  That  which  so  travels  is  not 
a  material  substance,  but  waves,  similar  in  some  re- 
spects to  the  waves  which  travel  on  water,  or  on  a 
stretched  rope.  That  which  distinguishes  red  light 
from  blue  light  is  the  length  of  the  wave,  or  the  num- 
ber of  complete  waves  executed  per  second.  The 
wave  lengths  of  visible  light  vary  from  about  0.0004 
millimeter  in  the  violet  to  0.0007  millimeter  in  the 
red;  and  the  corresponding  numbers  of  vibrations 
per  second  from  750  to  430  millions  of  millions.  But 
there  have  been  recognized  by  means  of  photography 

283 


THE   SUN 

rays  of  wave  length  only  0.0001  millimeter  and  wave 
frequency  3,000,000,000,000,000.  By  delicate  heat 
measuring  apparatus  rays  of  wave  length  0.06  milli- 
meter and  frequency*  5,000,000,000,000  have  been 
recognized.  All  this,  and  perhaps  a  wider  range  of 
spectrum,  is  probably  included  in  the  sun  beams  as 
they  leave  the  sun,  but  our  atmosphere  prevents 
some  of  the  shortest  and  longest  of  them  from  reach- 
ing the  surface  of  the  earth. 

Since  it  is  upon  the  supply  of  these  sun  rays  that 
heat,  light,  power,  and  the  growth  of  all  living  things 
upon  the  earth  depends,  the  measurement  of  the  in- 
tensity of  the  total  supply,  and  the  determination  of 
the  different  varieties  which  compose  it,  are  of  first- 
rate  interest  and  importance. 

We  measure  the  intensity  of  solar  radiation  by  the 
heat  which  it  will  produce  when  completely  absorbed 
on  a  surface  at  right  angles  to  the  rays.  A  conven- 
ient unit  for  measuring  solar  heating  is  the  calory  per 
square  centimeter  per  minute  (see  Chapter  II).  The 
maximum  intensity  of  solar  radiation  as  measured 
near  sea  level  at  Washington  when  the  sun  is  not  more 
than  45°  from  the  zenith  usually  ranges  from  1.15  to 
1.45  calories  per  square  centimeter  per  minute  on 
cloudless  days,  depending  on  the  clearness  and  dry- 
ness  of  the  air.  At  Mount  Wilson  in  California,  over 
one  mile  above  sea  level,  the  values  observed  range 
from  1.45  to  1.62  calories;  and  on  Mount  Whitney  in 
California,  nearly  three  miles  in  altitude,  the  ob- 
served values  reach  1.75  calories. 

284: 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

Fig.  60  shows  the  march  of  intensity  of  sun  rays 
during  the  forenoon  of  July  6, 1910,  on  Mount  Wilson. 
The  horizontal  scale  gives  zenith  distances,  the  verti- 


1.4 
1.2 
1.0 

OP 

^-  — 



—  -   ^ 

---  -^. 

-> 

~---~ 

•\. 

\ 

X 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

0.6 

\ 

\ 

0.2 
u 

\ 

1 

1 

y           \0'          20°          30°         40°         50*          6O'         70*         80"         9C 
ZENITH  DISTANCES. 

FIG.  60.  —  MARCH  OF  INSOLATION  (Mount  Wilson). 

cal  scale  calories  per  square  centimeter  per  minute. 
The  decrease  of  intensity  at  the  smallest  zenith  dis- 
tance observed  is  caused  by  increased  humidity,  due 
to  the  springing  up  of  a  sea  breeze  about  eleven 

285 


THE  SUN 

o'clock.     The  individual  observations  are  given  in 
Table  XVII,  page  287. 

Formerly  radiation  was  regarded  as  three  distinct 
entities,  namely:  actinic  or  chemical  rays;  visible  or 
light  rays;  obscure  or  heat  rays.  As  already  stated 
this  view  is  an  error  now  generally  abandoned,  and  all 
radiation  comprised  in  these  three  categories  is  rec- 
ognized as  of  the  same  fundamental  kind,  differing 
only  as  to  wave  length.  The  reader  will  therefore 
recognize  that  Table  XVIII  is  not  intended  to  re- 
vive this  ancient  classification,  but  only  to  fix  our 
ideas  of  the  amount  of  solar  radiation  found  in  the 
regions  (1)  where  ordinary  photographic  plates  are 
most  sensitive,  (2)  where  the  eye  is  the  most  sensitive, 
and  (3)  in  the  infra-red  spectrum.  These  facts  are 
given  for  the  beam  outside  the  earth's  atmosphere, 
and  as  it  reaches  Mount  Wilson  and  Washington 
under  different  angles  of  zenith  distance.  The  num- 
-bers  express  the  radiation,  within  the  stated  regions 
of  wave  length,  in  calories  per  square  centimeter  per 
minute.  The  zenith  distances  selected  are  0°,  60°,  70° 
32'  and  75°  32',  for  which  the  "air  masses"1  are  1,  2, 
3,  and  4. 

DEPENDENCE  OF  SOLAR  RADIATION  ON  AIR-MASS 

It  is  not  possible  to  express  satisfactorily  the  de- 
crease of  intensity  of  the  direct  solar  beam,  depending 

1  The  "air-mass"  is  the  ratio  of  the  length  of  the  path  of  the  sun's 
rays  in  the  atmosphere  to  the  corresponding  length  if  the  sun  were 
vertically  overhead.  It  is  closely  expressed  by  the  secant  of  the 
zenith  distance  for  zenith  distances  less  than  75°. 

286 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


8 


00 


CS3 


di 


II 


U 


287 


THE  SUN 


I 


^ 

i 

1 

S 

00 

o 

o 

d 

d 

00 

00 

f>^ 

CO 

fl 

CO 

Oi 

CO 

CO 

o 

^o 

0 

0 

d 

d 

13 

o 

CO 

00 

r_^ 

1 

* 

(M 

8 

i< 

T—  t 

CO 

8 

o 

d 

d 

,_! 

,_, 

1> 

(N 

oo 

i> 

II 

T—  t 

to 

GO 
CO 

CO 

s 

o 

d 

d 

i-H 

§ 

o 

i—  i 

CO 

CO 

G> 

o 

d 

CO 

d 

T—  1 

1 

CO 

i-H 

o 

1 

i 

p 

0 

d 

d 

^ 

3 

s 

p. 

oo 

1 

O 

" 

1-M 

^ 

^ 

. 

9S 

0 

d 

d 

TH 

7 

i 

CO 

S 

CO 

iO 

6 

0 

d 

d 

1—  1 

c^ 

| 

00 

53 

Tf 

1—  I 

*o 

cO 

CO 

0 

d 

d 

TH 

tO 

0 

CO 

i-H 

CO 

C^l 

CO 

K*> 

CO 

T—  1 

lO 

!>• 

^^ 

| 

o 

d 

d 

_! 

'Jc 

8 

J2 

CO 

oo 

^ 

(M 

(M 

CO 

to 

"2 

0 

d 

d 

T—  1 

O 

* 

§ 

§. 

(M 

g 

^ 

II 

C^l 

00 

s 

d 

d 

d 

rH 

o 
II 

^ 

^ 

S 

& 

s 

0 

d 

00 

d 

_; 

1 

3 

E 

1 

a 

g 

10 

o 

•< 
^ 

4.2 

3.S 

8 

8 

s 

3 

3 

3 

.8 

^ 

^ 

8 

4.O 

4.O 

SLO 

3.0 

288 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

on  the  decreasing  elevation  of  the  observing  station. 
For  the  lower  layers  of  the  air  contain  a  load  of  dust 
and  water  vapor  which  changes  in  quantity,  quality, 
and  distribution  from  hour  to  hour  and  day  to  day. 
In  short,  the  variation  of  the  solar  beam  in  the  lower 
atmosphere  does  not  proceed  according  to  any  fixed 
law  of  relation  to  the  barometric  pressure. 

As  for  the  change  of  the  intensity  of  the  direct  solar 
beam  for  different  zenith  distances  of  the  sun,  that 
may  be  well  expressed  by  Bouguer's  exponential  for- 
mula, e  =  e0  a  sec-  z,  as  explained  in  Chapter  II,  pro- 
vided we  deal  with  homogeneous  rays  (rays  which 
are  practically  of  one  wave  length),  and  observe  them 
at  a  single  station  on  a  clear  day.  If  we  imagine  the 
atmosphere  to  be  made  up  of  a  great  number  of  shells 
concentric  with  the  earth,  and  the  shells  of  such  thick- 
ness as  to  contribute  equal  amounts  to  the  barometric 
pressure,  each  of  the  upper  shells  will  transmit  to  the 
shell  next  below  practically  an  unchanging  fraction  of 
the  intensity  the  shell  receives  of  a  homogeneous  ray. 
But  when  the  ray  reaches  a  layer  within  one  or  two 
miles  of  sea  level  the  fraction  transmitted  continually 
decreases  from  shell  to  shell  owing  to  the  increasing 
load  of  dust  carried  by  the  lower  layers. 

The  total  thickness  of  the  atmosphere  necessary  to 
be  considered  as  affecting  solar  radiation  is  less  than 
one  hundred  miles,  and  is  so  small  compared  with  the 
earth's  radius  that  the  shells  may  be  regarded  as  prac- 
tically parallel  planes,  except  when  we  deal  with  rays 
entering  the  atmosphere  at  very  great  zenith  dis- 

289 


THE   SUN 

tances.  Atmospheric  refraction,  too,  may  be  neg- 
lected in  these  computations  for  rays  whose  zenith 
distance  is  not  above  75°.  Hence  we  may  assume 
that  for  zenith  distances  less  than  75°  the  ratio  of  the 
length  of  the  path  of  the  ray  in  each  shell  to  the  thick- 
ness of  the  shell  is  constant,  and  equal  to  the  secant 
of  the  zenith  distance.  Under  these  restrictions  (as 
shown  in  Chapter  II)  the  exponential  formula  of 
Bouguer  serves  to  determine  the  intensity,  e,  of  mono- 
chromatic rays  at  different  zenith  distances,  even 
though  we  do  not  know  the  change  of  transmission 
from  layer  to  layer.  For  as  the  sun  rises  higher  and 
higher  the  thickness  in  every  layer  changes  in  the 
same  proportion.  In  thought  we  may  go  even  further, 
and,  with  the  sun  in  the  zenith,  imagine  that  the 
thickness  in  every  layer  should  be  reduced  simultane- 
ously in  equal  proportions  until  no  air  remains.  In 
other  words,  we  can,  after  the  secant  reaches  its  min- 
imum value,  unity,  substitute  another  function  of  the 
quantity  of  air  in  each  shell,  which  we  imagine  to  be 
decreased  in  equal  proportion  in  all  layers  until  no 
more  atmosphere  is  left.  Thus  we  may  determine 
the  intensity,  eoy  which  our  monochromatic  ray  would 
have  outside  the  earth's  atmosphere. 

The  quantity,  a,  which  appears  in  the  formula,  is 
the  fraction  of  the  intensity  outside  the  earth's  at- 
mosphere which  remains  in  the  beam  as  it  reaches  the 
observer  at  the  earth's  surface.  This  quantity  is 
called  the  atmospheric  transmission  coefficient.  It 
differs  with  the  altitude  of  the  observer  and  the  clear - 

290 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

ness  of  his  sky.  It  differs  also  for  rays  of  different 
colors;  increasing,  generally  %  as  we  pass  from  short 
wave  lengths  to  longer  ones.  There  are,  however,  cer- 
tain rays  which  suffer  powerful  selective  absorption  in 
the  gases  and  vapors  of  the  earth's  atmosphere,  and 
for  such  rays  the  transmission  coefficients  are  very 
small.  Absorption  bands  play  a  very  great  part  in 
the  red  and  infra-red  spectrum,  where  the  bands  of 
oxygen,  water  vapor,  and  carbon  dioxide  are  principal- 
ly found.  This  is  made  clear  in  the  accompanying 
illustration,  Fig.  61,  which  shows  two  successive  ob- 
servations made  on  Mount  Wilson  by  the  bolometer 
of  the  relative  intensity  of  the  rays  in  the  solar  spec- 
trum of  a  60°  flint-glass  prism.  At  places  marked  *  the 
sun  rays  were  cut  off  so  as  to  give  the  base  line,  or  line 
of  zero  radiation.  At  places  marked  |  the  sun  rays 
were  altered  in  intensity  so  as  to  keep  the  curve  within 
the  bounds  of  the  plate.  The  heights  above  the  base 
line  are  proportional  to  the  energy  of  the  spectrum 
rays.  The  length  is  proportional  to  the  prismatic  de- 
viation. Fraunhofer  lines  show  as  depressions  of  the 
curve.  Prominent  Fraunhofer  lines  are  indicated  by 
their  letters.  These  energy  curves,  or  holographs, 
were  made  on  Mount  Wilson  as  a  part  of  a  series  of  six 
such  curves  obtained  at  different  solar  zenith  dis- 
tances in  a  single  forenoon.  They  were  made  to  de- 
termine the  transmission  of  the  atmosphere  at  all 
parts  of  the  spectrum.  From  such  observations  the 
distribution  of  solar  radiation  as  it  would  be  outside 
of  our  atmosphere  is  computed.  We  have  studied  in 

291 


THE   SUN 


o 

1-g 


11 

fc  "o 

£      O 

II 

*    -f— 


292 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

Chapter  III  the  significance  of  such  work  in  regard 
to  the  sun's  temperature. 

We  could  not  determine  the  intensity  outside  the 
atmosphere  if  the  transparency  of  the  air  varied  much 
during  the  several  hours  required  to  complete  the 
series  of  holographs.  Fortunately  there  is  the  follow- 
ing criterion  for  the  excellence  of  any  given  day  in 
this  respect:  In  the  course  of  the  usual  reductions, 
logarithms  of  the  heights  above  the  base-line  (corre- 
sponding to  intensities  of  radiation  at  given  wave 
lengths  are  plotted  against  zenith  distances.  The 
results  should  show  straight  lines.  Fig.  62,  p.  294, 
shows  how  well  this  test  is  met  by  the  Mount  Wilson 
conditions.  The  tangent  of  the  inclination  of  such 
lines  gives  the  logarithm  of  the  transmission  at  ver- 
tical sun,  which  we  have  called  a.  Values  of  a  for 
a  given  wave  length  are  of  course  greater  for  Mount 
Wilson  than  for  Washington.  By  dividing  the  aver- 
age Washington  values  by  those  for  Mount  Wilson 
we  obtain  the  average  transmission  of  the  mile  of  air 
nearest  sea  level,  as  it  is  above  Washington.  We 
shall  see  in  the  following  tables  that  the  loss  in  passing 
through  this  last  mile  of  the  air  is  almost  the  same  as 
the  entire  loss  above  Mount  Wilson. 

Bouguer's  formula  is  not  exactly  applicable  to  pyr- 
heliometric  measurements  of  the  total  radiation  (or 
summation  of  all  rays  of  all  wave  lengths)  of  the  sun. 
It  fails  because  rays  suffer  unequal  extinction  in  the 
atmosphere,  some  being  almost  completely  extin- 
guished in  the  upper  air  owing  to  the  action  of  water 

293 


THE  BUN 

vapor  and  other  selective  absorbents.     Hence  for 
these  rays  the  intensity  at  the  earth's  surface  does 


T* 

\ 

\ 

\ 

\ 

\ 

\    t 

\ 

\ 

x 

N 

> 

\ 

:2J79 

C 

\ 

,2.928 

X 

x 

\ 

\ 

• 

x 

X 

\ 

\ 

X 

N 

\ 

\ 

1 

7* 

^ 

. 

°\ 

\ 

p 

V 

^ 

K 

\J 

L\ 

:*I28 

e 

N 

K, 

\ 

\ 

^3.120 

""^t" 

r-     a 

—  0  

•^>-- 

—  C 

-  —  o 

0- 

^X 

\4 

^ 

-  —  4 

-^ 

~~--^ 

^•1.12 
~V.7&< 

ffodtu, 

^94r 


3.6 
FIG.  62. — ATMOSPHERIC  TRANSMISSION  PLATS. 

not  alter  much  with  the  zenith  distance.    Neverthe- 
less the  exponential  formula  holds  approximately 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

even  for  total  radiation,  except  that  the  logarithmic 
plats  like  those  given  in  Fig.  62  are  slightly  curved, 
and  if  continued  on  in  straight  lines  to  zero  atmos- 
pheric thickness  they  fall  below  the  real  intensity  of 
total  solar  radiation  outside  the  earth's  atmosphere, 
as  obtained  from  spectrum  observations.  Pouillet, 
however,  determined  transmission  coefficients  for  the 
total  solar  radiation,  and  was  thereby  led  to  his  cele- 
brated value  1.76  calories  per  square  centimeter  per 
minute  for  the  solar  constant  of  radiation.  Radau, 
and  later  Langley,  showed  clearly  that,  on  account  of 
the  differences  of  transmission  for  rays  of  different 
wave  lengths,  we  must  observe  the  transmission  of 
each  color  by  itself,  and  determine  what  the  intensity 
of  each  separate  color  would  be  outside  the  atmos- 
phere. Langley  first  applied  this  procedure  experi- 
mentally. Following  his  method  we  may  sum  up  the 
area  included  under  the  solar  spectrum  energy  curve 
outside  the  earth's  atmosphere,  and  compare  it  with 
the  area  for  the  corresponding  curve  at  zero  zenith 
distance  of  the  sun.  Thus  we  may  find  the  actual 
vertical  transmission  of  the  atmosphere  for  the  total 
radiation.  Knowing  by  measurements  of  the  pyr- 
heliometer  the  intensity  of  total  radiation  for  any  ob- 
served zenith  distance  we  can  determine  how  many 
heat  units  the  area  of  the  corresponding  spectrum- 
energy  curve  represents.  Summing  up  in  similar 
terms  the  area  as  it  would  be  outside  the  earth's  at- 
mosphere we  may  obtain  the  true  " solar  constant." 

21  295 


THE   SUN 


TRANSMISSION  OF  THE  ATMOSPHERE 

In  the  following  table  there  are  given  the  mean 
results  for  the  vertical  transmission  of  total  solar 
radiation,  according  to  observations  of  the  Astro- 
physical  Observatory  of  the  Smithsonian  Institution. 

TABLE  XIX. — Transmission  for  total  solar  radiation 


PLACE 

Washington 

Mount 
Wilson 

Mount 
Whitney 

True  transmission  

0.699 

0.817 

0.896 

Apparent  transmission  

0.787 

0.894 

0.960 

Table  XX,  opposite,  gives  the  atmospheric  trans- 
mission for  vertical  rays,  and  for  the  zenith  distances 
whose  secants  are  two  and  three,  respectively,  for  rays 
of  various  wave  lengths. 

THE  SOLAR  CONSTANT  OF  RADIATION 

From  the  mean  results  of  the  Washington  observa- 
tions of  1902  to  1907,  the  Mount  Wilson  observations 
of  1905  to  1910,  and  Mount  Whitney  observations  of 
1909,  1910,  all  corrected  to  the  absolute  scale  of  heat,1 
the  total  intensity  of  solar  radiation  outside  the 
earth's  atmosphere  at  the  earth's  mean  distance  from 
the  sun  (called  the  " solar  constant"  of  radiation), 

1  Values  published  in  Volume  II  of  the  Annals  of  Astrophysical 
Observatory  of  the  Smithsonian  Institution  were  given  on  a  pro- 
visional scale  of  pyrheliometry  differing  about  five  per  cent,  from  the 
true  one,  and  are  here  given  as  reduced  to  true  calories. 

296 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


s 


CO  !>•  Q  CO 


oo 


o 


o 


O"ti  t^O 
!>•  t^*  CO  OS 

OS  OS  00  00 

o'ddd 


Tt<  c^i  oo  o 

to  to  co  os 
os  os  oo  oo 

dodo 


os  oo  i>  oo 

dddd 


os  oo  t^-  oo 
dddd 


l>»  (^  o^  C^ 
T^  CO  CO  t^* 

00  00  CO  t^ 

o'ddo 


t^  oo  to  to 

00  CO  CO  <N 
t^  t^  to  1^ 

dddd 


odo 


lOCO 

do 


t^»  CO  <N 

O5  00  00 


OS  OS  00  00 

dddd 


(M  O  tO  rH 

CO  t^  CO  CO 
OS  Os  00  00 

dddd 


OS  OS  00  00 

dddd 


OSfN  d 
"  to  Os 
t~  l> 

dddd 


OCO<M  c^ 

i—  i  O  O  O5 


»O 

oo 


0  (M  COO 

i-H  CO  O5  ^O 


t^  O5  oo  co 

i-H  00  O  C5 


OS  O  CO  CO 

rH  CO  00  OJ 

dddd 


Tt^CO 

do 


Ss 


O  <N  iO  TH 

OOOSt^-00 

dddd 

t^  <M  •<*!  OS 


OS  OS  00  00 

dddd 


o 


CO  Tf  <N  iO 

T-H  <N  tOO 
O^  O^  CO  Is* 

dddd 


00  00  tO  t^ 

dddd 


CO 

CO 


O5  Tfi 

Tji  <M 

CO  O 


(M  rH  t^ 

1>  CO  >O 
>O  <N  ^t1 


COr-l 

oo  oo  10  oo 

Tt<  CO  i—  i  CO 


o 


o 


li^ 


fill 


II' 

M*H, 


It-^f 


297 


THE  SUN 


as  expressed  in  calories  per  square  centimeter  per 
minute  is  as  follows : 


PLACE 

Washington 

Mount  Wilson 

Mount 
Whitney 

Date  

1902-1907 

1905 

1906 

1908 

1909 

1910 

1909 

1910 

Observations. 

44 

59 

62 

113 

95 

28 

1 

3 

Mean  solar 
constant.  .  . 

1.960 

1.925 

1.921 

1.929 

1.896 

1.914 

1.959 

1.956 

In  1909  and  1910  observations  were  made  simul- 
taneously on  Mount  Wilson  (elevation  one  mile)  and 
Mount  Whitney  (elevation  nearly  three  miles)  by 
Smithsonian  observers,  with  the  following  results: 


DATE 

1909  Sept.  3 

1910  Aug.  12 

Aug.  13 

Aug.  14 

Mount  Wilson  

1.943 

1.943 

1.924 

1.904 

Mount  Whitney  

1.959 

1.979 

1.933 

1.956 

We  see  that  notwithstanding  the  differences  in  al- 
titude of  the  observing  stations,  and  the  differences 
of  atmospheric  transmission  above  them,  there  is 
good  agreement  between  the  computed  values  of  the 
" solar  constant"  of  radiation.  Prior  to  1905  this 
quantity  was  in  great  doubt,  as  numbers  ranging 
from  1.76  to  4.10  had  been  given  for  it,  and  the  ac- 
cepted value  then  was  3.0.  The  opinion  now  seems  to 
prevail  that  no  considerable  change  from  the  Smith- 
sonian result  of  about  1.95  calories  per  square  centi- 

298 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

meter  per  minute  will  come  from  future  experiment- 
ing. 

Expressed  in  another  way,  the  measurements  indi- 
cate that  if  the  sun's  rays  could  be  completely  em- 
ployed to  melt  ice  exposed  continuously  to  them  at 
right  angles,  they  would  suffice  to  melt  a  layer  426 
feet  thick  in  a  year.1  Such  a  layer  at  the  earth's  mean 
distance,  if  it  entirely  surrounded  the  sun,  would 
weigh  4  X  1025  (4  followed  by  25  ciphers)  tons,  and 
the  complete  melting  of  it  each  year  would  represent 
as  many  heat  units  as  the  burning  of  4  X  1023  tons 
of  anthracite  coal.  This,  then,  is  a  measure  of  the 
sun's  yearly  output  of  radiation. 

THE  LIGHT  OF  THE  SKY 

It  must  not  be  inferred  from  the  tables  given  on 
a  preceding  page  that  only  81.7  per  cent  of  the  sun's 
radiation  reaches  the  Mount  Wilson  level  at  vertical 
sun.  That,  to  be  sure,  is  the  average  result  for  the 
direct  solar  beam,  but  the  sky  supplies  an  appreciable 
addition  of  indirect  rays  even  on  Mount  Wilson.  At 
sea  level  the  sky  light  is  a  still  more  considerable  por- 
tion of  the  total  radiation,  but  as  yet  not  very  ex- 
actly measured.  The  relative  brightness  of  the  sun 
and  sky  differs  greatly  according  to  the  manner  in 
which  the  rays  are  received.  Owing  to  the  great  ex- 
tent of  the  sky,  it  is  not  possible,  when  receiving  rays 

1  As  the  earth  has  four  times  the  area  of  its  cross-section,  we  may 
say  that  the  sun's  rays  are  capable  of  melting  an  ice  shell  covering  the 
earth  to  an  average  thickness  of  106 . 5  feet  annually. 

299 


THE   SUN 

simultaneously  from  its  whole  extent,  to  have  them 
all  fall  at  right  angles  to  the  absorbing  surface. 
Hence  the  sky  light  is  at  a  disadvantage  with  respect 
to  sunlight,  unless  we  observe  the  brightness  from 
every  part  of  the  sky  by  itself  and  then  sum  up  the 
results.  From  bolometric  measurements  of  1905 
and  1906,  made  by  the  Smithsonian  observers,  and 
reduced  in  this  manner,  it  appears  that  the  total  sky 
radiation  on  Mount  Wilson  computed  at  normal  in- 
cidence, and  including  all  wave  lengths,  is  from  eleven 
to  twenty  per  cent  of  the  total  direct  sun  radiation. 
Both  sun  and  sky  rays  are  in  this  estimate  supposed 
to  be  received  at  right  angles  to  the  absorbing  sur- 
face, and  the  sun  to  be  not  over  50°  from  the  zenith. 
The  percentages  depend  on  the  clearness  of  the  sky, 
increasing  with  the  haziness.  If  we  make  the  as- 
sumption that  the  sky  shines  on  a  horizontal  surface, 
and  the  sun  upon  a  surface  normal  to  the  beam,  these 
percentages  become  5.2  and  7.7.  If  both  sun  and  sky 
rays  are  supposed  to  shine  on  a  horizontal  surface, 
the  ratio  varies  of  course  greatly  from  hour  to  hour. 
Professor  Exner  has  derived  formula  for  the  rela- 
tive brightness  of  the  sun  and  sky  on  the  hypothesis 
that  the  sky  light  is  all  due  to  scattering  from  parti- 
cles which  are  small  as  compared  with  the  wave 
length  of  the  rays.1  He  has  found  it  necessary  to  make 
some  rather  rough  simplifying  assumptions.  Never- 
theless his  computations  fall  in  pretty  well  with  such 

1  Sitzungsbericht,  d.     K.  Akad.  d.  Wissen.,  Wien.,  M.  N.  Klasse. 
CXVIII,  Ha,  1909. 

300 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


observations  as  are  available.  In  the  following 
tables  taken  from  Exner's  publication,  z  is  the  zenith 
distance  of  the  sun,  H  the  intensity  of  sky  light  and  S 
that  of  sunlight,  both  being  measured  on  a  horizontal 
surface.  At  normal  incidence  outside  the  atmos- 
phere the  intensity  of  sunlight  is  taken  as  unity.  The 
quantity  p  is  the  transmission  coefficient  of  the 
atmosphere  above  the  observer  for  a  vertical  ray. 
From  Smithsonian  observations  we  see  that  p  =  0.6 
would  correspond  to  wave  length  0.43/4  (violet)  at 
Washington  0.35/4  (ultra-violet)  at  Mount  Wilson. 
Correspondingly,  for  p  =  0.75  we  have  0.59/z,  (yellow) 
at  Washington  and  0.41//,  (violet)  at  Mount  Wilson. 

TABLE  XXI. — Sunlight  and  sky  light.     (Exner.) 


p  =  0.6 

p  =  0.75 

Z 

H 

S 

S  +  H 

S 
H 

H 

S 

S  +  H 

S 
H 

80° 

0.241 

0.009 

0.250 

0.04 

0.136 

0.032 

0.168 

0.24 

70° 

0.245 

0.077 

0.322 

0.31 

0.138 

0.147 

0.285 

1.06 

60° 

0.252 

0.180 

0.432 

0.72 

0.141 

0.282 

0.423 

2.00 

50° 

0.259 

0.289 

0.548 

1.12 

0.146 

0.408 

0.554 

2.79 

40° 

0.268 

0.394 

0.662 

1.47 

0.151 

0.528 

0.679 

3.50 

30° 

0.276 

0.484 

0.760 

1.75 

0.155 

0.625 

0.780 

4.03 

20° 

0.281 

0.547 

0.828 

1.95 

0.158 

0.693 

0.851 

4.38 

10° 

0.285 

0.582 

0.867 

2.04 

0.160 

0.731 

0.891 

4.57 

0° 

0.288 

0.600 

0.888 

2.08 

0.162 

0.750 

0.912 

4.63 

The  change  of  H  with  the  zenith  distance  of  the 
sun  is  not  as  great  in  these  tables  as  it  should  be. 
This  appears  from  the  following  measurements  of 
Roscoe  for  which  we  may  assume  p  =0.6. 

The  units  employed  by  Roscoe  are  not  the  same  as 
301 


THE   SUN 
TABLE  XXII. — Sunlight  and  sky  light.     (Roscoe.) 


z 

80°  9' 

70°  19' 

58°  46' 

47°  47' 

36°  51' 

25°  46' 

H 

0.038 

0.062 

0.100 

0.115 

0.126 

0.138 

S 

0.000 

0.023 

0.052 

0.100 

0.136 

0.221 

those  employed  by  Exner,  so  that  for  easier  compari- 
son the  results  of  Roscoe  may  be  multiplied  by  2  or 
2.5.  As  there  is  much  difference  for  different  days 
and  for  different  stations  in  results  of  this  kind,  Ex- 
ner's  computations  seem  to  be  near  enough  at  least 
for  giving  a  general  idea  of  the  state  of  affairs.  Indeed 
the  following  summary,  which  I  translate  from  Wies- 
ner's  description  of  his  photographic  observations  of 
light  received  on  horizontal  surfaces,1  fits  Exner's 
results  for  short  wave  lengths  very  well : 

"The  direct  sunlight,  which  is  sometimes  twice  the 
intensity  of  diffused  light,  may  also  sink  to  zero.— 
For  solar  altitudes  less  than  19°  (z  =  71°)  the  chem- 
ical intensity  of  the  sunlight  as  compared  with  dif- 
fused daylight  is  negligible.  With  increasing  solar 
altitude  the  intensity  of  the  direct  sunlight  gains  in 
comparison  with  the  diffused  light.  The  solar  alti- 
tude for  which  S  =  H  seems  not  to  be  constant  even 
for  apparently  clear  sky,  and  for  one  and  the  same 
station.  For  cloudless  sun  the  equality  of  direct  and 
diffused  light  occurs  generally  when  the  solar  alti- 
tude is  about  57°  (z  =  33°),  yet  with  clear  sky  it  was 


Vienna  Academy. 


Denkschriften,  Bd.  64,  1897. 
302 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


observed  once  at  33°  (z  =  57°).  Since  the  intensity 
of  the  direct  beam  may  reach  twice  that  of  the  dif- 
fused, the  total  combined  chemical  effect  may  be 
threefold  that  of  the  diffused  light." 

Exner  also  gives  computations  of  the  relative 
amounts  of  combined  direct  and  diffused  light  receiv- 
able on  vertical  surfaces  facing  respectively  South, 
North,  West,  East,  compared  with  the  amounts  re- 
ceived on  a  horizontal  surface.  The  southern  expos- 
ure only  of  the  vertical  surface  is  supposed  to  receive 
some  direct  sunlight,  designated  by  2.  V8,  VN,  Vw, 
VE  designate  the  diffused  illumination  of  the  vertical 
surface  toward  the  South,  North,  West,  and  East. 
S  and  H  have  their  former  meanings. 

TABLE  XXIII. — Sky  light  on  a  vertical  surface.    (Exner,  Schramm.) 


Computed,   p=0.8 

Observed  by  W.  Schramm 

z 

*+Va 

VN 

Vw          VE 

2  +  Vs 

VN 

Vw 

VE 

S  +  H 

S  +  H 

S  +  H     S  +  H 

S  +  H 

S  +  H 

S  +  H 

S  +  H 

85° 

1.43 

0.537 

0.461 

2.73 

0.560 

0.542 

0.604 

75 

2.21 

0.263 

0.241 

3.41 

0.268 

0.397 

0.386 

65 

1.69 

0.148 

0.146 

1.81 

0.258 

0.331 

0.351 

55 

1.25 

0.099 

0.104 

1  32 

0.147 

0.223 

0.204 

45 

0.92 

0.075 

0.083 

0.976 

0.118 

0.195 

0.175 

35 

0.68 

0.062 

0.071 

0.749 

0.091 

0.131 

0.139 

It  is  not  probable  that  the  influence  of  the  direct 
sunlight  was  wholly  absent  in  Schramm's  VE  and  Vw 
observations.  Apart  from  these  there  is  a  pretty 
good  agreement  of  observed  and  computed  reslilts, 
as  the  following  summary  also  indicates. 

303 

V 


THE   SUN 


Ratios  of  average  vertical  illumination  to  that  of 
the  North. 


2  +  Vs 

VN 

Vw 

VE 

v 

»  mean 

Observed  

7.64 

1  00 

1.26 

1.29 

2.80 

Computed  .....*... 

6.91 

1.00 

0.94 

0.94 

2.45 

Exner  computes  from  p  =  0.6  and  z  =  40°,  the 
following : 


S  +  H 

3  +  Vs 

VN 

V  mean 

S  +  H 

3  +  Vs 

v  w  —  VE 

v 

V  mean 

VN 

0.662 

0.480 

0.106 

0.120 

0.207 

3.2 

4.5 

As  we  have  stated  at  some  length  in  Chapter  VI, 
Schuster  has  employed  Lord  Rayleigh's  theory  of 
the  scattering  of  light  by  particles  small  compared 
with  the  wave  lengths,  to  compute  the  transmission 
of  the  direct  beam  of  sunlight.  He  assumes  that  the 
loss  in  the  atmosphere  is  wholly  from  the  scattering 
caused  by  the  molecules  of  the  air.  He  finds  close 
agreement  between  the  computed  and  observed  re- 
sults for  excellently  clear  days  at  Mount  Wilson  and 
Washington.  This  seems  to  indicate  that  the  dust 
load  of  the  atmosphere  plays  a  subordinate  part  in 
affecting  the  solar  radiation  on  the  best  days,  and 
that  under  such  conditions  as  are  found  ordinarily 
at  Mount  Wilson,  and  occasionally  at  Washington, 
nearly  all  the  light  of  the  sky  is  due  to  the  diffuse 

304 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


reflection  or  scattering  of  sun  rays  by  the  molecules 
of  air.  Somewhat  similar  conclusions  have  been 
reached  still  more  recently  by  Natanson,  except  that 
he  considers  the  matter  from  the  point  of  view  of  the 
electron  theory. 

The  light  of  the  sky  is  very  much  richer  in  the  vio- 
let rays  than  that  of  the  direct  solar  beam.  From 
Smithsonian  experiments  the  following  values  are 
taken  for  different  wave  lengths,  assuming  about 
equal  intensities  in  the  extreme  red  for  a  sunbeam 
and  a  skybeam,  and  giving  the  approximate  normal 
spectral  distribution  for  both  as  observed  at  the  sur- 
face of  Mount  Wilson. 

TABLE  XXIV. — Sun  and  sky  light.     Relative  brightness  for  different 
wave  lengths  on  Mount  Wilson. 


WAVE  LENGTH     -» 

0.422/* 

0.4.57M 

0.491,* 

0.556,* 

0.614M 

0.660/* 

Sunlight;  z  -  50°... 
Skylight  
Ratio  

186 
1,194 
6.92 

232 
986 
4.25 

227 

701 
3.09 

211 
395 
1.87 

191 
231 
1.21 

166 
174 
1.05 

At  the  sea-level,  especially  in  cities  and  other  dusty 
localities,  the  proportion  of  blue  in  sky  light  is  usu- 
ally much  less  than  that  given  above;  for  particles 
large  as  compared  with  the  wave  length  of  light,  such 
as  occur  in  dust,  do  not  act  in  the  same  way  as  small 
particles  and  molecules.  Large  particles,  by  reflect- 
ing sunlight,  tend  rather  to  diminish  than  to  increase 
the  relative  proportion  of  the  intensity  due  to  rays  of 
short  wave  length. 

Skylight  is  brightest  near  the  sun  and  near  the  hori- 

305 


THE   SUN 


€ 


i    12 


OI^ 

co  o 


oo 


306 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


zon.  The  results  shown  in  Table  XXV  on  the  total 
radiation  are  from  bolometric  measurements  at 
Mount  Wilson,  and  at  Flint  Island,  a  coral  island 
in  the  Pacific,  near  the  equator,  lying  400  miles 
northwest  of  Tahiti. 

The  measurements  are  reduced  as  if  the  sun  were 
in  the  zenith. 

Summing  up  columns  (V)  and  (VI)  we  find  the 
average  brightness  of  a  portion  of  the  sky  equal  in 
angular  area  to  the  sun  as  compared  with  the  bright- 
ness of  the  sun:  First  as  received  on  a  surface  at 
right  angles  to  the  beam  in  both  cases;  second  with 
the  skylight  received  on  a  horizontal  surface  and  the 
sunlight  received  normally.  The  measurements  in- 
clude all  the  rays  transmissible  by  a  glass  plate  three 
millimeters  thick.  The  results  are  as  follows: 

TABLE  XXVI. — -Ratio  of  total  radiations:  Sky  to  sun 


STATION 

FOR  EQUAL  AREAS 

FOR  WHOLE  SKY 

Normal 
incidence 

Sky  on 
horizontal 

Normal 
incidence 

Sky  on 
horizontal 

Flint  Island  

636X10-8 

302X10-8 

0.67 

0.32 

Mount  Wilson  

176X10-8 

69X10-8 

0.18 

0.072 

Thus  according  to  these  measurements  (which  how- 
ever are  not  sufficiently  numerous  or  exact)  at  sea 
level  the  sky  furnishes  to  a  horizontal  surface  thirty- 
two  per  cent  as  much  radiation  as  the  direct  high  sun. 
At  1,800  meters  elevation  only  7.2  per  cent. 

307 


THE  SUN 

THE  DEPENDENCE  OF  THE  EARTH'S  TEMPERATURE 
ON  RADIATION 

The  temperature  of  the  earth  seems  to  be  main- 
tained at  present  almost  wholly  by  the  absorption  of 
solar  radiation.  It  is  thought  by  some  that  the 
earth's  temperature  is  rising  slowly.  If  so,  this 
would  indicate  that  the  sum  total  of  the  earth's  sup- 
plies of  heat  exceeds  its  losses.  But  this  change  of 
temperature,  if  real,  is  so  exceedingly  slow  that  we 
may  practically  say  that  the  earth's  heat  income  and 
heat  outgo  balance.  The  outgo,  neglecting  the  rela- 
tively trifling  effects  of  vegetable  and  other  storage 
processes,  is  made  up  wholly  of  the  earth's  radiation 
of  long- wave  rays  to  space.  It  has  been  shown  that 
the  absorptive  effects  of  atmospheric  water  vapor, 
carbonic  acid  and  ozone  combined  prevent  nearly 
or  quite  nine-tenths  of  the  rays  which  are  emitted 
at  the  earth's  surface  from  escaping  directly  to 
space. 

Hence  the  earth's  effective  radiating  layer  may  be 
regarded  as  situated  in  the  atmosphere,  and  as  being 
chiefly  the  water  vapor  layers  at  several  miles  ele- 
vation, whose  average  temperature  is  about  — 10°  C. 
The  still  higher  situated  effective  radiating  layers  of 
carbonic  acid  and  ozone  gases,  whose  average  temper- 
atures reach  as  low  as  — 60°  C,  also  radiate  freely  in  a 
few  limited  regions  of  spectrum.  We  shall  not  be 
far  astray,  therefore,  if  we  regard  the  average  tem- 
perature of  the  earth's  radiating  layer  as  not  above 

308 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

260°  of  the  absolute  centigrade  scale.1  The  constitu- 
ents of  this  radiating  layer,  namely,  water  vapor,  car- 
bonic acid  and  ozone,  owing  to  their  powerful  selec- 
tively absorbing  properties  for  rays  proper  to  this 
temperature,  must  also  be  nearly  perfect  radiators 
for  the  same  temperature.  Admitting  that  their 
radiating  power  is  perfect,  the  emission  of  the  as- 
sumed radiating  layer  at  260°  C.  absolute  is,  by 
Stefan's  law  (see  Chapter  II) ,  about  0.34  calories  per 
square  centimeter  per  minute. 

In  order  to  sustain  this  average  rate  of  loss  of  heat 
over  the  whole  surface  of  the  earth,  solar  radiation, 
shining  effectively  over  only  the  area  of  the  earth's 
cross-section,  must  be  absorbed  at  four  times  this 
rate,  or  1.36  calories  per  square  centimeter  per  min- 
ute. Of  the  energy  represented  by  the  solar  con- 
stant (1.95  calories)  about  thirty-five  per  cent  is  re- 
flected away  according  to  the  Smithsonian  determi- 
nation of  the  earth's  "  albedo. "  The  remainder  is  1.27 
calories,  and  nearly  suffices  to  furnish  the  heat  above 
computed  as  lost  from  the  earth.  The  difference 
(0.09  calories)  may  possibly  mean  that  an  appreciable 
quantity  of  heat  is  furnished  by  terrestrial  sources, 
such  as  radio-active  processes.  However  it  seems 
quite  reasonable  to  suppose  that  the  difference  may 
be  accounted  for  (1)  by  assuming  that  the  earth's 
effective  radiating  layer  is  not  a  perfect  radiator,  so 
that  its  radiation  falls  short  of  the  0.34  calories  per 
square  centimeter  per  minute  which  a  perfect  radi- 

1  Water  freezes  at  273°  of  this  scale,  and  boils  at  373°. 
309 


THE  SUN 

ator  at  260°  C.  absolute  would  emit:  or  (2)  that  the 
effective  radiating  temperature  is  below  260°  C. 
absolute. 

The  surface  temperature  of  the  earth  reaches  310° 
absolute  C.  in  the  tropics,  and  at  the  poles  falls  as 
low  as  220°,  and  its  effective  mean  temperature  is 
287.2°  absolute  C.  or  +  14.2  C.  It  exceeds  the  tem- 
perature of  the  radiating  layer  by  over  25°.  This  is 
in  large  measure  for  the  same  reason  that  the  gar- 
dener's hot  beds,  or  the  steamfitter's  asbestos-covered 
pipes,  exceed  the  temperature  of  their  surroundings. 
For  the  sun's  rays  shine  through  the  atmospheric 
vapors  readily,  and  warm  the  earth's  surface.  The 
escape  of  its  heat,  as  we  have  seen,  is  hindered  by  the 
atmosphere.  Hence  the  earth's  surface  temperature 
rises  sufficiently  to  force  a  flow  of  heat  out  to  the 
effective  radiating  layer.  If  it  was  not  for  the  blank- 
eting effect  of  the  water  vapor  of  the  atmosphere,  the 
earth's  mean  surface  temperature  would  probably  be 
nearly  20°  C.  below  freezing,  providing  the  reflecting 
power  of  the  earth  was  not  changed.  But  if  there 
was  no  water  vapor  in  our  air,  the  sun's  rays  would 
reach  the  earth's  surface  with  at  least  ten  per  cent 
greater  intensity  on  cloudless  days  than  they  do  now. 
Since  clouds  would  then  be  absent  there  would  be 
about  1.75  calories  instead  of  1.27  as  now  available 
to  warm  the  earth.  Consequently,  the  earth's  mean 
temperature,  if  water  was  absent,  would  be  about 
277°  absolute  or  +  4°  C.  But  there  would  then  be 
a  much  greater  range  of  temperature  between  night 

310 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


and  day  and  between  summer  and  winter  than  there 
is  now. 

Upon  the  moon  there  is  no  atmosphere,  and  by  the 
observations  of  Lord  Rosse,  of  Langley  and  of  Very, 
the  moon's  sunlit  surface  falls  from  about  the  tem- 
perature of  boiling  water  nearly  to  that  of  liquid  air 
within  the  short  duration  of  a  total  lunar  eclipse. 
Quite  otherwise  is  the  state  of  affairs  on  the  earth. 
In  the  following  table  is  given  the  yearly  average  of 
the  daily  range  of  temperatures  which  occurs  at  sev- 
eral stations  on  the  earth. 

TABLE  XXVII. — Yearly  means  and  mean  daily  temperature  depar- 
tures.    (Centigrade.) 


HOUR      -» 

Lati- 
tude 
/ 

Mid- 
night 

2 

4 

6 

8 

10 

12 

Timbuctu  
Port  au  Prince.  .  . 

16°49'N 
18°34'N 

-4°.l 
-2°.  6 

-5°.  6 
-3°.  2 

-6°.  8 
-3°.  7 

-7°.  7 
-3°.  8 

-2°.  8 
-0°.6 

+3°.  2 
+2°.  9 

+6°.  9 

+4°.  7 

HOUR      -» 

Noon 

2 

4 

6 

8 

10 

12 

Mean 

Timbuctu 

+6°  9 

+8°  5 

+7°  4 

+3°  4 

-0°  1 

-2°  4 

-4°  1 

29°.  2 

Port  au  Prince  .  .  . 

+4°.  7 

+4°.  5 

+3°.l 

+l°.l 

-0°.8 

-1°.3 

-2°.  6 

25°.  9 

Even  a  polar  night  of  five  months'  duration  in 
which  the  sun  is  continuously  below  the  horizon  pro- 
duces no  such  range  of  temperature  on  the  earth  as  a 
total  lunar  eclipse  of  a  few  hours'  duration  does  upon 
the  moon.  Witness  the  following  mean  temperatures : 

Fort  Conger.     Latitude  81°  44'.     Temperatures  Centigrade. 


Jan. 

Feb. 

Mar. 

April 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

-39°.0 

^0°.l 

-33°.5 

-25°.3 

-10°.0 

+0°.4 

+2°.8 

+1°.0 

-9°.0 

-22°.7 

-30°.9 

-33°.4 

311 


THE   SUN 

These  examples  indicate  how  slowly  the  temperature 
of  the  earth  falls  towards  the  absolute  zero  when  the 
solar  radiation  is  utterly  cut  off.  The  delay  cannot 
be  attributed  to  the  influence  of  the  inner  heat  of  the 
earth.  From  the  rise  of  temperature  at  increasing 
depths  in  the  earth  (about  1°  C.  in  28  meters)  taken 
in  connection  with  the  observed  conductivity  of  rock 
(about  0.0042  calories  per  second  per  centimeter  cube) 
it  is  calculated  that  the  heat  supplied  to  the  surface 
from  within  is  but  0.00010  calories  per  square  centi- 
meter per  minute,  which  would  suffice  to  keep  a  per- 
fect radiator  at  only  34°  absolute  (Centigrade)  tem- 
perature and  could  not  be  expected  to  keep  the  earth's 
surface  above  40°  absolute,  or  -233°  C.  Even  this  is 
far  above  what  the  moon  and  all  the  stars  combined 
could  do  to  supply  the  place  of  the  sun. 

The  following  table1  of  the- mean  monthly  temper- 
atures (Centigrade)  gives  some  idea  of  the  yearly 
ranges  of  temperatures  on  the  earth  at  various  sta- 
tions in  the  Northern  Hemisphere.  Several  pairs  of  sta- 
tions at  nearly  the  same  latitude,  but  one  inland,  the 
other  oceanic,  are  contrasted  to  show  the  influence  of 
the  oceans  in  reducing  fluctuations  of  temperatures. 

The  reader  will  notice  how  much  smaller  are  the 
yearly  ranges  of  temperatures  for  oceanic  stations 
than  those  for  the  inland  stations.  It  is  also  appar- 
ent that  the  yearly  range  increases  with  the  latitude. 
This  is  due  in  part  to  the  growing  disparity  of  the 

1  The  data  are  taken  mainly  from  various  publications  of  J.  v. 
Hann. 

312 


THE  RUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


STATION 

Ver- 
khoy- 
ansk 

Fort 
Conger 

St. 
Louis 

(U.S.A.) 

P. 

Delgada 
(Azores) 

Tim- 
buctu 

Port 
au 
Prince 

Bogota 

Jaluit 
(Mar- 
shall 
Islands) 

Latitude  N 

67°34'° 

81°44'° 

38°38' 

37°45' 

16°49'° 

i8°34'° 

4°31' 

5°55' 

Elevation 
(meters) 

173 

20 

250 

36 

2660 

3 

January.  .  . 
February.  . 
March  
April  
May. 

-f>l°.0 
-4o°.3 
-32°.5 
-13°.7 
+  2°.0 

-39°.0 
-40°.  1 
-33°.  5 
-25°.3 
-10°.0 

-  0°.8 

+  1°.7 
+  6°.2 
+13°.4 
-j-18°.8 

+14°.l 
+13°.9 
+14°.l 
+  lo°.4 
+16°.6 

+21°.8 
+23°.8 
+28°.  1 
+32°.5 
-j-35°.0 

+24°.  1 
+24°.6 
+25°.  1 
+25°.9 
+26°  0 

14°.2 
I4°.4 
14°.8 
14°.7 
14°  8 

27°.  1 
27°.2 
27°.0 
26°.9 
26°  9 

June.  .  . 

+12°.3 

+  0°.4 

+24°.0 

+18°.9 

+34°.2 

+27°.  1 

14°  5 

26°  8 

July  
August.  .  .  . 
September. 
October  .  .  . 
November. 
December  . 

+15°.5 
+10°.l 
+  2°.o 
-15°.0 
-37°.8 
-47°.0 

+  2°.8 
+  1°.0 
-  9°.0 
-22°.7 
-30°.9 
-33°.4 

+26°.0 
+24°.9 
+20°.8 
+14°.2 
+  6°.4 
+  2°.0 

+21°.3 
+22°.0 
+20°.9 
+18°.9 
+16°.9 
+15°.l 

+32°.7 
+31°.l 
+31°.8 
+31°.0 
+26°.8 
+21°,4 

+27°.6 
+27°.3 
+26°.7 
+26°.3 
+25°.6 
+24°.4 

14°.l 
13°.9 
13°.9 
14°.4 
14°.7 
14°.5 

26°.8 
26°.9 
26°.9 
27°.  1 
27°.  1 
27°.0 

Yearly 
Range 

60°.  5 

42°.9 

26°.8 

8°.l 

13°.G 

3°.o 

0°.9 

0°.4 

Yearly 
Mean 

-16°.7 

-20°.0 

+  13°.l 

+  17°.3 

+29°.2 

+25°.9 

14°.4 

27°.0 

longest  and  shortest  days  at  higher  latitudes,  and  in 
part  to  the  more  rapid  change  in  the  intensity  of 
illumination  with  change  of  zenith  distance  of  the 
sun  at  high  latitudes.  At  the  equator  the  days  and 
nights  are  always  equal,  and  the  secant  of  the  zenith 
distance  of  the  noonday  sun  varies  only  from  1  to 
0.917.  At  latitude  45°  N.  the  length  of  day  varies 
from  eight  hours,  thirty-four  minutes  to  fifteen  hours, 
twenty-six  minutes,  and  the  secant  of  the  noonday 
zenith  distance  from  0.930  to  0.366.  The  value  of 
the  secant  of  the  zenith  distance  influences  the  result 
in  two  ways,  first,  as  it  measures  the  length  of  the 
path  of  the  rays  in  the  air,  second,  as  it  measures 
their  weakening  on  a  horizontal  surface  in  conse- 
quence of  obliquity. 

313 


THE  SUN 

A  minor  influence  which  affects  the  yearly  march 
of  the  solar  radiation  is  the  change  of  the  sun's  dis- 
tance from  the  earth.  This  causes  an  increase  of 
nearly  seven  per  cent  in  the  earth's  heat  supply  from 
July  to  January;  and  combined  with  the  sun's  march 
in  declination  it  produces  two  maxima  arid  two 
minima  of  radiation  in  the  tropics.  At  Bogota 
temperature  maxima  occur  March  to  May  and  Oc- 
tober to  December — minima  August  to  September, 
and  January,  all  occurring  a  little  after  the  corres- 
ponding maxima  and  minima  of  radiation. 

By  taking  the  three  factors  of  solar  distance, 
obliquity  and  the  daily  duration  of  sunlight  into 
account,  formulae  have  been  devised  for  comput- 
ing the  " effective  insolation"  as  it  is  sometimes 
called.  This  is  the  intensity  of  the  uniform  beam, 
which  if  received  continuously  at  normal  incidence 
would  yield  an  equal  supply  of  radiation  to  that 
which  is  really  effective  on  a  horizontal  surface. 
In  such  computations  atmospheric  losses  are  usu- 
ally neglected,  but  on  the  other  hand  diffused  sky 
radiation  is  also  neglected.  We  may  also  imagine 
an  hypothetical  earth  equal  in  size  and  similar  in 
motions  to  the  real  earth,  but  a  perfect  absorber  and 
radiator;  thin  as  an  egg  shell;  perfectly  conducting 
of  heat  from  east  to  west,  but  perfectly  non-conduct- 
ing from  north  to  south.  The  temperature  of  such  a 
structure  can  be  computed  for  all  times  and  latitudes 
by  Stefan's  law  (see  Chapter  II).  When  such  com- 
puted temperatures  are  compared  with  those  actu- 

314 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

ally  observed  on  the  earth,  it  is  found  that  no  real 
stations  show  as  great  yearly  ranges  of  temperature 
as  the  corresponding  hypothetical  ones.  For  Tim- 
buctu  and  some  other  desert  stations  the  observed 
range  is  over  half  the  computed.  For  average  inland 
stations  the  ratio  is  about  three-tenths;  for  average 
coast  stations,  one-fifth;  for  average  island  stations, 
one- twelfth;  at  Apia,  in  Samoa,  only  one  twenty- 
fifth.  For  the  hypothetical  earth  the  percentage 
change  of  absolute  temperature  is  everywhere  one- 
fourth  of  the  percentage  change  in  solar  radiation 
which  would  cause  it. 

The  accompanying  illustration,  Fig.  63,  gives  the 
march  of  the  " effective  insolation"  at  the  north  lat- 
itudes 17°  40'  and  5°  10',  and  also  the  yearly  change 
of  temperature  at  Timbuctu  (16°  49'  N.),  Port  au 
Prince  (18°  34'  N.),  Bogota  (4°  31')  and  Jaluit  (5°  55'). 
The  curves  show  how  much  the  effect  of  solar  change 
may  be  modified  by  local  conditions,  and  especially 
how  considerable  are  the  delays  which  occur  at 
oceanic  stations  between  solar  causes  and  their  ter- 
restrial temperature  effects.  Thus,  while  at  Tim- 
buctu, an  inland  station,  the  maximum  and  minimum 
temperatures  attend  closely  the  minima  of  effective 
insolation,  they  are  so  far  delayed  that  the  maximum 
temperature  occurs  three  months  after  the  insolation 
is  at  its  maximum  at  St.  Louis,  Senegambia,  a  coast 
station  nearly  west  of  Timbuctu,  Such  facts  should 
be  taken  into  consideration  when  seeking  by  studying 
temperature  statistics  to  determine  if  fluctuations  of 

315 


THE   SUN 


JAN.  APR.  JULY  OCT. 

FIG.  63. — INSOLATION  AND  TERRESTRIAL  TEMPERATURES. 
316 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

solar  radiation  probably  occur.  Long  solar  periods 
like  the  eleven-year  sun-spot  period  may  be  expected 
to  affect  the  temperatures  of  all  terrestrial  stations 
nearly  simultaneously.  Not  so  temporary  solar 
changes  of  a*  few  days  or  months.  These  could  be 
expected  to  show  their  influence  only  at  inland  sta- 
tions, and  preferably  cloudless  ones.  The  lag  of  tem- 
perature minima  behind  the  inducing  solar  radiation 
minima  is  about  twenty  days  for  average  continental 
stations,  only  ten  days  for  particularly  favorable 
ones,  but  reaches  two  months  or  more  in  the  case  of 
many  island  ones. 

FLUCTUATION  OF  SOLAR  EMISSION 

Numerous  attempts  have  been  made  to  see  if  ter- 
restrial temperatures,  by  their  departures  from  the 
normals,  indicate  fluctuations  of  the  sun's  emission 
of  radiation.  Koppen  concluded  from  such  investi- 
gations, published  in  1873,  1880,  and  1881,  that  the 
earth's  temperature  is  higher  at  sun-spot  minimum 
than  at  sun-spot  maximum.  This  conclusion  is  con- 
firmed by  Stone,  Gould,  Nordmann,  Newcomb, 
Abbot  and  Fowle,  Arctowski,  Bigelow,  and  others. 
Taking  a  general  view  of  their  results  with  those  of 
Koppen,  we  may  conclude  that  for  a  change  of  100 
sun-spot  numbers  of  Wolf's  scale,  which  is  about  the 
average  range  of  sun-spot  activity,  there  is  a  change 
of  the  mean  temperature  of  the  earth  of  about  0.7°  C . 
The  cause  of  this  cannot  be  in  the  mere  darkening  of 
the  disk  of  the  sun  by  the  areas  covered  by  the  spots, 

317 


THE  SUN 

for  if  they  were  perfectly  black  all  over  the  change  of 
solar  radiation  corresponding  to  100  sun-spot  num- 
bers would  be  only  -— :,  which  is  not  \  of  what  is  nec- 
500 

essary  to  produce  the  observed  change -of  tempera- 
ture. There  must  therefore  be  other  changes  of  the 
sun  (or  possibly  in  the  earth's  atmosphere  or  in  the 
intervening  space),  as  yet  not  understood,  which  at- 
tend the  increase 'of  sun  spots  and  which  exceed  in 
their  effective  reduction  of  solar  radiation  the  direct 
influence  of  the  darkness  of  the  spots  themselves. 

It  is  only  within  the  last  five  years  that  there  have 
been  direct  measurements  of  the  solar  radiation  suf- 
ficiently complete  and  accurate  to  show  whether 
there  are  frequent  changes  of  the  sun's  emission  suf- 
ficiently large  to  affect  the  earth's  temperature  notice- 
ably. The  reader  might  be  inclined  to  suppose  that 
a  mere  analysis  of  the  deviations  from  normal  tem- 
peratures at  numerous  stations  would  be  sufficient  to 
practically  verify  or  disprove  frequent  variability 
of  the  sun.  Indeed  there  are  found  numerous  in- 
stances of  departures  from  normal  temperatures 
which  seem  to  indicate  something  of  the  kind,  but  yet 
the  evidence  of  different  localities  is  so  contradictory 
and  confusing  that  careful  meteorologists  reserve  an 
opinion  on  the  matter.  Recently  the  idea  has  gained 
some  adherents  that  a  few  per  cent  of  increase  of 
solar  radiation  during  a  period  of  several  months  need 
not  necessarily  affect  the  temperatures  of  all  stations 
on  the  earth  in  the  same  direction,  but  might  make 

318 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

some  warmer  and  others  cooler  temporarily,  so  that 
the  effect  may  thus  be  obscured.  This  contradiction 
of  effects,  as  these  observers  think,  may  be  due  to 
changes  of  cloudiness  and  of  the  circulation  of  the 
atmosphere.  Statistical  studies  of  temperatures  by 
Arctowski,  Bigelow,  and  others  support  the  view  that 
certain  large  regions  are  on  the  whole  balanced  in 
their  temperature  changes,  so  that  the  same  dis- 
turbance which  warms  one  region,  cools  another,  as 
if  there  were  waves  of  effect  superposed  upon  the 
earth.  Apparently  the  meteorological  condition  of 
the  earth  is  so  complicated  by  the  relative  configura- 
tion of  land  and  sea,  cloudy  and  clear  areas  and  hot 
and  cold  regions  that  we  cannot  expect  to  determine 
solar  changes  with  certainty  by  climatic  investiga- 
tions, and  must  rather,  turning  the  matter  about, 
first  determine  the  solar  changes  by  direct  observa- 
tions, and  then  search  out  their  terrestrial  effects. 

Up  to  1905  the  measurements  of  solar  radiation 
made  in  different  countries  by  different  investigators 
were  so  much  at  variance  as  to  make  it  seem  highly 
unlikely  that  sufficiently  accurate  knowledge  of  the 
solar  emission  could  be  obtained  to  lead  to  the  dis- 
covery of  variability  of  the  sun.  But  the  Smithson- 
ian observations  at  Washington,  Mount  Wilson,  and 
Mount  Whitney  as  given  on  a  previous  page  are  so 
highly  concordant,  and  seem  to  be  so  probably  com- 
petent to  fix  the  intensity  of  the  solar  radiation  out- 
side the  atmosphere  within  about  one  per  cent,  that 
there  now  seems  to  be  a  good  prospect  of  discovering 

319 


THE  SUN 

fluctuations  of  solar  radiation  if  they  exceed  one  per 
cent  from  the  mean  value.  Heretofore  the  observa- 
tions have  not  been  kept  up  continuously,  but  ef- 
forts are  being  made  to  remedy  this  defect  by  the 
establishment  of  one  or  more  additional  "  solar  con- 
stant "  observing  stations. 

From  measurements  of  the  Smithsonian  observers 
on  Mount  Wilson  during  six  months  of  each  of  the 
years  of  1905,  1906,  1908,  and  1909,  as  given  in  Fig. 
64,  it  appears  probable  that  fluctuations  of  the  sun  at 
irregular  intervals  of  several  days,  and  sometimes  of 
several  months,  are  not  uncommon.  Apparently  the 
amplitude  of  such  changes  sometimes  reaches  ten 
per  cent,  and  seems  frequently  to  reach  from  three  to 
five  per  cent.  But  notwithstanding  that  the  reality 
of  these  changes  is  attested  by  various  evidences, 
such  as  .the  continuity  of  a  change  during  several 
days  of  consecutive  observing,  which  the  reader  can 
see  for  himself  in  the  observations  of  1908  and  1909, 
Fig.  64,  and  the  fact  that  the  fluctuation  of  radiation 
depending  on  the  yearly  'change  of  the  solar  distance 
can  be  easily  recognized,1  though  it  amounts  to  but 
three  per  cent  during  the  six  months  covered  by  the 
Mount  Wilson  work,  yet  the  supposed  solar  variabil- 
ity can  hardly  be  said  to  be  conclusively  shown  until 
another  station  as  well  equipped  as  Mount  Wilson 
supports  the  conclusion  by  simultaneous  measure- 
ments. 

If  occasional  variations  of  ten,  or  even  five,  per 

1  The  values  platted  in  Fig.  64  are  reduced  to  mean  solar  distance. 

320 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


321 


THE   SUN 

cent,  shall  be  found  to  exist  in  the  solar  radiation, 
they  may  well  be  expected  to  produce  noticeable 
effects  on  the  earth's  climate.  We  have  seen  that  it 
is  difficult,  if  not  impossible,  to  seek  the  cause  from 
the  observed  climatic  effects,  owing  to  their  great 
complexity.  But  when  the  variations  of  the  sun 
become  accurately  observed,  and  thus  the  action  of 
the  cause  is  known,  the  tracing  of  the  climatic  effects 
will  be  a  matter  of  great  interest  and  importance. 
Indeed,  some  comparisons  of  temperature  statistics 
with  the  " solar  constant"  values  have  been  made  al- 
ready, and  indicate  a  probable  connection  between 
the  two.  But  much  more  work  is  needed  for  certainty. 
A  solar  change  of  five  per  cent  continued  for  six 
months  might  well  alter  the  mean  temperature  of 
inland  stations  by  2°  C.,  or  3.6°  F.,  and  this  would 
make  the  difference  between  an  unusually  hot  and 
an  unusually  cold  season.  Its  influence  in  temperate 
zones  on  the  length  of  season  favorable  to  vegetable 
growth  would  be  very  noticeable,  as  will  be  more 
clearly  shown  in  the  following  chapter. 

GEOLOGICAL  TEMPERATURES 

It  is  generally  believed  that  from  the  Cambrian  to 
the  Pleiocene  a  genial  climate  usually  prevailed  at 
the  poles,  and,  moreover,  without  evidences  of  ex- 
traordinarily high  temperatures  at  the  equator.  This 
state  of  affairs  seems  to  be  inconsistent  with  the  view 
that  the  sun  controlled  the  temperatures  then  in  the 
same  manner  that  it  does  at  present. 

322 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

On  the  other  hand,  the  sub-tropical  glaciation1 
which  interrupted  this  condition  of  uniform  temper- 
atures during  the  Permian  period  seems  inconsistent 
with  the  present  value  of  the  solar  constant  of  radia- 
tion. If  the  constant  is  two  calories  per  square  cen- 
timeter per  minute,  the  average 2  insolation  per 
square  centimeter  per  minute  in  latitude  28°  is  0.55 
calories.  The  largest  possible  part  of  this  supply 
would  be  lost  for  purposes  of  heating  if  the  earth  was 
completely  cloudy.  According  to  observations  of 
Smithsonian  observers  this  maximum  possible  loss, 
at  this  tropical  latitude  of  high  sun,  would  be  less  than 
forty- two  per  cent,  leaving  at  least  0.32  calories  ab- 
sorbed. This  remainder  would  maintain  a  perfect 
radiator  at  254.3°  absolute  Centigrade,  or  -18.7  C. 
If  the  radiating  layer  were  not  perfectly  " black"  it 
would  have  a  higher  temperature  than  this,  and  any 
contribution  of  heat  from  the  earth's  interior  would 
also  tend  to  raise  its  temperature.  Under  the  per- 
fectly cloudy  condition  we  are  considering,  the  upper 
part  of  the  cloud  layer  would  absorb  most  of  the  ab- 
sorbable  radiation  from  the  sun,  but  its  own  outward 
radiation  would  be  restrained,  then  as  now,  by  the 
water  vapor,  carbon  dioxide  and  ozone  lying  higher. 
Accordingly  there  would  be  a  " blanket"  or  " hot- 
house" effect  similar  to  that  which  now  exists,  and 
which  now  raises  the  surface  temperature  of  the 
earth  nearly  30°  C.  above  the  temperature  of  the 

1  See  description  quoted  near  the  end  of  Chapter  VI. 

2  For  night  and  day  for  the  whole  year. 

323 


THE  SUN 

radiating  layer.  Hence  we  may  suppose  that  the 
upper  layer  of  the  clouds  would  also  have  been  nearly 
30°  C.  above  the  temperature  ( — 18.7)  which  has  just 
been  computed  for  a  perfectly  radiating  layer,  mak- 
ing the  cloud  temperature  about  +  10°  C.  Accord- 
ing to  Kirchhoffs  law  the  earth  within  such  a  mantle 
would  also  be  at  the  same  temperature,  or  higher  if 
warmed  at  all  by  internal  heating.  Thus  it  seems 
unlikely  that  a  perfectly  cloudy  earth  could  have 
been  glaciated  at  latitude  28°  while  a  solar  constant 
of  2.0  calories  prevailed.  If  the  earth  was  not  per- 
fectly cloudy  the  conditions  would  have  been  less 
favorable  for  glaciation  and  more  like  those  of  the 
present  time.1  The  matter  of  Permian  tropical  glaci- 
ation is  still  more  perplexing  when  we  consider  that 
there  was  no  glaciation  simultaneously  in  temperate 
and  polar  regions. 

Reverting  now  to  the  hypothesis  called  (A)  in 
Chapter  VI :  If  in  Permian  times  and  still  earlier  the 
solar  radiation  alone  was  far  too  little  to  maintain  the 
surface  of  the  earth  or  its  clouds  above  freezing,  then 
glacial  conditions  would  have  been  produced  in  those 
times  by  a  very  moderate  rise  of  land  level.  For  sup- 
posing conduction  from  within  and  radio-activity  to 
have  been  considerable  sources  of  earth  heat,  and  the 
outer  layers  of  the  clouds  not  greatly  warmed  by  the 
sun,  the  thickness  of  the  water  vapor  bearing  stratum 

1  The  preceding  argument  does  not  tend  to  show  that  high  tropical 
mountains  might  not  be  glaciated.  The  reasons  for  cold  tempera- 
tures on  high  mountains  have  already  been  explained. 

324 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 


would  have  been  much  less  then  than  now,  since  the 
vertical  temperature  gradient  of  the  atmosphere 
would  have  been  far  more  rapid.  Accordingly  a 
much  less  degree  of  elevation  than  now  would  suffice 
to  reach  levels  of  comparatively  free  radiation  to 
space,  and  air  currents  cold  enough  to  cause  snow. 

On  this  hypothesis  we  may  represent  the  contribu- 
tory influences  which  maintained  the  terrestrial  cli- 


TIME. 

FIG.  65. — HYPOTHETICAL  TEMPERATURE  DIAGRAM. 

mate  in  geologic  time  by  the  accompanying  Fig.  65. 
The  upper  curved  line  indicates  the  earth's  tempera- 
ture, the  lower  curved  line  what  it  would  have  been 
if  the  sun  had  been  the  sole  contributor  of  heat  to  the 
earth.  No  attempt  is  made  to  draw  the  figure  to 
scale  either  in  time  or  temperature,  but  only  to  illus- 
trate the  idea  proposed. 

The  grave  difficulty  with  our  hypothesis  of  a  low 
intensity  of  solar  radiation  in  early  geological  periods 
is  of  course  the  question  how  the  earth's  surface  tem- 

325 


THE  SUN 

perature  was  generally  maintained  above  freezing. 
We  do  not,  to  be  sure,  argue  that  no  heat  at  all  came 
from  the  sun,  but  only  that,  while  increasing,  it  had 
not  in  Permian  times  reached  perhaps  three-fourths 
its  present  intensity.  No  difficulty  arises  in  sup- 
posing that  in  the  very  earliest  geological  times  the 
earth's  own  heat  sufficed  completely  to  maintain  its 
surface  temperature.  The  difficulty  lies  in  supposing 
that  the  earth  could  have  still  contributed  appreci- 
ably after  the  enormous  lapse  of  ages,  estimated 
roughly  at  50,000,000  years,  to  Permian  times.  This 
difficulty  appears  to  me  insuperable. 

We  will  turn  now  to  hypothesis  (B)  stated  in  Chap- 
ter VI.1  According  to  Laplace's  theory  of  the  origin 
of  the  solar  system  we  are  to  suppose  that  when  the 
earth  was  formed  the  sun  was  expanded  so  as  almost 
to  fill  the  orbit  of  the  earth.  Other  nebular  hypothe- 
ses recognize  the  probable  existence  of  much  meteoric 
nebulosity  in  the  solar  system  at  that  epoch.  It  is  to 
be  supposed  that  at  that  stage  the  sun  itself  was  a 
combined  structure  of  nebulosity  and  condensation, 
such  perhaps  as  we  see  in  the  Pleiades  stars  (see 
Plate  XIX),  and  was  not  spherical,  but  still  its 
polar  diameter  was  very  much  greater  than  now. 
Under  such  circumstances  the  sun,  with  its  outlying 
appendages,  as  viewed  from  the  earth  would  sub- 
tend a  great  part  of  a  hemisphere,  so  that  its  rays 
would  be  nearly  equally  diffused  all  over  the  earth's 
surface.  Such  a  state  of  affairs  would  have  pro- 

1  This  hypothesis  was  suggested  by  Chamberlin  about  1898. 
326 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

moted   the   uniformity   of    temperatures   we   have 
already  noted. 

Besides  this,  the  intensity  of  the  sun's  radiation 
must  have  been  very  small;  for  otherwise,  coming 
from  so  large  an  angle,  it  would  have  melted  the 
earth.  If  the  rays  came  to  the  earth  from  a  hemi- 
sphere, and  the  sun  then  filled  the  earth's  orbit,  the 
total  output  of  the  sun  to  space  need  not  have  ex- 
ceeded half  its  present  amount.  Thus  our  hypothe- 
sis may  release  us  from  the  difficulty  of  understanding 
how  the  sun  could  have  radiated  so  long  without  the 
temperature  of  the  earth  showing  marked  change. 
For  in  early  times  we  imagine  the  solar  radiation  of 
very  slight  intensity,  but  the  angle  subtended  by  the 
sun  very  large.  With  increasing  solar  density  the 
angle  diminished,  but  the  output  of  solar  radiation 
increased.  We  may  even  suppose  considerable  neb- 
ulosity existed  all  about  the  earth,  and  that  this  neb- 
ulosity, by  reflecting  solar  rays,  and  by  sending  some 
long  wave  rays  of  its  own,  helped  to  diminish  the 
rigor  of  the  demands  which  geology  induces  us  to 
make  on  the  sun  in  ancient  times.  Hence  our  hypo- 
thesis (B)  relieves  us  of  the  difficulty  of  the  problem 
of  the  supply  of  solar  energy  during  the  enormous 
lapse  of  geological  time. 

As  regards  the  possibility  of  tropical  glaciation: 
We  may  suppose  that  the  full  maintenance  of  ordi- 
nary temperatures  required  formerly,  as  it  does  now, 
the  cooperation  of  the  blanketing  effect  of  the  water 
vapor  of  the  earth's  atmosphere;  and  that  in  addition 
23  327 


THE  SUN 

to  this  the  earth's  internal  sources  of  heat  were  then 
of  some  appreciable  importance  in  maintaining  its 
surface  temperature.  The  earlier  the  period  we  con- 
sider, the  greater  we  may  suppose  the  contribution 
of  the  earth's  own  heat,  and  the  less  the  requirement 
of  the  sun.  But  we  may  assume  that  all  three  fac- 
tors, solar  radiation,  terrestrial  conduction  and  the 
blanketing  effect  of  the  earth's  atmosphere  were  re- 
quired to  maintain  genial  temperatures  in  the  Per- 
mian period.  As  we  have  assumed  that  solar  radia- 
tion was  nearly  uniformly  distributed  over  the  earth's 
surface,  because  of  the  large  angle  subtended  by  the 
sun  and  of  the  reflection  of  radiation  by  still  existing 
outlying  nebulosity  about  the  earth,  glaciation  at  the 
tropics  was  accordingly  no  more  difficult  to  bring  on 
then  than  glaciation  elsewhere.  Hence,  a  regional  ele- 
vation of  land  areas,  or  any  other  means  of  bringing 
about  a  reduction  of  the  efficiency  of  the  atmosphere 
as  a  blanket,  in  any  locality,  would  have  produced 
local  glaciation.  Snow  and  ice  once  formed,  would 
help  to  perpetuate  themselves  by  their  high  reflecting 
power. 

There  are  several  ways  in  which  the  efficiency  of 
the  atmosphere  as  a  blanket  may  be  altered.  One  of 
these  is  by  a  considerable  reduction  of  the  atmos- 
pheric humidity,  and  this,  though  somewhat  unfav- 
orable to  great  rainfall,  would  still  be  in  line  with  the 
known  pronounced  aridity  of  the  Permian  period. 
But  decreased  humidity  generally  brings  with  it  de- 
creased cloudiness,  which  permits  more  solar  radi- 

328 


THE  SUN  AS  THE  EARTH'S  SOURCE  OF  HEAT 

ation  to  be  received,  and  thereby  tends  to  raise  in- 
stead of  depress  temperatures.  Hence  it  may  be 
that  we  are  to  look  for  the  supposed  regional  altera- 
tion of  temperature  rather  in  a  considerable  increase 
of  cloudiness,  due  to  change  in  the  relative  arrange- 
ment of  land  and  oceans. 

4 

But,  in  whichever  of  the  several  ways  suggested, 
or  in  still  others,  the  local  decrease  of  temperature 
might  have  been  brought  about,  the  hypothesis  (B) 
is  evidently  highly  favorable  to  the  explanation  of 
tropical  glaciation,  since  it  makes  it  just  as  easy  to 
produce  ancient  glaciations  in  the  tropics  as  in  polar 
regions.  When  the  Pleistocene  period  arrived  we 
assume  that  the  sun  had  so  far  shrunk  that  its  influ- 
ence was  then,  as  now,  zonal.  We  may  further  sup- 
pose, if  we  choose,  that  the  sun's  radiation  was  less 
then  than  now,  and  that  this  combined  with  other 
causes  to  produce  the  Pleistocene  glaciation.  It  is 
well  known  that  one  of  these  causes  was  a  consider- 
able elevation  of  the  glaciated  areas. 

Our  hypothesis  (B)  seems  to  relieve  us  at  once  of 
three  formidable  difficulties,  and  enables  us  to  under- 
stand: 1.  How  the  sun  has  continued  to  suffice  for 
terrestrial  needs  throughout  geological  time.  2.  Why 
earlier  geological  periods  were  characterized  by  uni- 
formity of  climate  irrespective  of  latitude.  (3)  How 
it  was  possible  to  have  tropical  glaciation  at  all  dur- 
ing the  Permian  and  earlier  epochs;  but  especially 
without  evidence  of  simultaneous  overwhelming  gla- 
ciation over  all  the  temperate  and  polar  zones  of  the 

329 


THE  SUN 

earth.  Unfortunately  the  most  favoring  foundation 
of  hypothesis  (B),  namely,  the  Laplacian  nebular 
hypothesis,  is  now  strongly  attacked  on  dynamical 
grounds,  as  we  shall  see  in  Chapter  X.  The  substi- 
tute proposed  by  Chamberlin  and  Moulton  seems 
less  adapted  to  the  line  of  explanation  just  given. 
For  it  leaves  little  opportunity  for  the  development 
of  solar  heat  by  contraction,  and  besides  does  not 
permit  us  to  assume  so  widely  spread  sources  of  the 
earth's  supply  of  radiation  in  ancient  times.  Never- 
theless Chamberlin  himself  put  forth  the  rudiments 
of  hypothesis  (B)  about  twelve  years  ago.  We  need 
not  yet  despair  that  a  nebular  hypothesis  may  be 
proposed  as  suitable  to  our  purpose  as  to  other  re- 
quirements.1 

1  A  reference  to  See's  views  is  made  in  Chapter  X. 


CHAPTER  VIII 

THE    SUN'S   INFLUENCE    ON  .PLANT   LIFE 

Plant  Requirements. — The  Assimilation  of  Carbon  by  Autotrophic 
Plants. — Etiolation. — Plant  Geography. — Light  Requirements 
of  Plants. — Heliotropism. — Plants  as  Energy  Accumulators. 

THE  vegetable  kingdom  varies  so  widely  in  forms, 
habits,  and  every  characteristic  of  its  members,  that 
the  reader  must  not  expect  in  this  chapter  a  discus- 
sion of  all  the  sun's  functions  with  respect  to  all 
plants.  But  the  higher  plants,  such  as  everybody 
sees  in  the  forests  and  fields,  and  which  provide  not 
only  food  for  man  and  beast,  but  countless  materials 
for  building  and  the  arts,  are  directly  and  indirectly 
dependent  in  many  interesting  ways  on  the  sun's 
radiation.  The  subject  of  plant  growth  is  so  full  of 
cases  of  extraordinary  adaptations  that  it  is  hard  to 
avoid  digressing  from  the  story  of  purely  solar  influ- 
ences to  speak  of  some  of  these;  and  perhaps  readers 
may  pardon  a  few  such  excursions  from  the  main 
highway  of  our  subject. 

PLANT  REQUIREMENTS 

The  higher  plants  require  carbon,  oxygen,  hydro- 
gen, nitrogen,  sulphur,  phosphorus,  potassium,  cal- 
cium, magnesium,  and  iron.  Living  vegetation 

331 


THE  SUN 

tains  a  very  high  percentage  of  water;  but  both  of  its 
constituents,  oxygen  and  hydrogen,  also  enter  largely 
into  more  complex  compounds  with  carbon.  As  re- 
gards their  methods  of  obtaining  carbon,  plants  are 
classified  in  three  groups:  (1)  Auto  trophic,  or  the 
self-nourishing,  which  obtain  it  through  their  leaves, 
under  the  influence  of  light,  from  the  carbonic  acid 
gas  of  the  air.  (2)  Saprophytes,  or  scavengers,  which 
take  it,  in  part  at  least,  through  their  roots  from  de- 
caying vegetable  and  animal  organisms.  (3)  Para- 
sitic plants,  which  take  nearly  all  their  nourishment 
from  living  vegetation  on  which  they  fasten  them- 
selves. We  shall  practically  confine  our  attention  to 
the  first  class,  and  when  we  use  the  term  plant  for 
short  in  what  follows,  we  shall  mean  generally  auto- 
trophic  plants. 

All  plants  are  largely  composed  of  water  and  most 
of  them  employ  it  profusely  in  their  vital  actions. 
A  large  birch  tree,  according  to  Von  Hohnel's  figures, 
may  send  into  the  air  through  its  leaves  in  one  day 
eighty  pounds  of  water,  which  it  has  gathered  mainly 
from  the  soil  by  its  roots.  If  200  such  trees  grew  on 
an  acre  their  water  output  in  a  season  would  perhaps 
reach  1,500  tons.  While  not  all  trees  and  plants  are 
proportionally  as  free  in  using  water  as  this,  or  indeed 
can  be,  still  they  all  require  it,  and  depend  on  the  sun 
not  only  to  keep  water  in  the  liquid  form,  but  also  to 
promote  the  atmospheric  circulation  which  promotes 
the  rainfall.  These  two  functions,  first  maintaining 
a  proper  temperature,  second  inducing  a  sufficient 

333 


THE  SUN'S   INFLUENCE   ON   PLANT   LIFE 


rainfall,  are  in  this  age  almost  wholly  solar.    Formerly 
it  may  possibly  have  been  otherwise. 

From  determinations  of  Konig  we  learn  the  follow- 
ing percentage  compositions  of  some  of  the  common 
vegetables. 

TABLE  XXVIII. — Composition  of  food  products 


Non-nitrog- 

Water 

Fat  Ether 
Extract 

Nitrogenous 
Material 

enous  (as 
Carbohy- 

Wood 
Fibre 

Ash 

drates) 

Wheat  (grain)  .  . 

13.65 

1.75 

12.35 

67.91 

2.53 

1.81 

Potato  tubers  .  . 

75.48 

0.15 

1.95 

20.69 

0.75 

0.98 

Beetroot  

87.61 

0.11 

1.09 

9.26 

0.98 

0.95 

Lettuce  (leaves) 

94.33 

0.31 

1.41 

2.19 

0.73 

1.03 

The  various  chemical  substances  mentioned  above 
as  plant  requirements,  and  also  some  others,  occur  in 
weak  solution  in  the  water  which  plants  so  plenti- 
fully absorb  through  their  roots.  We  cannot  enter 
into  that  profoundly  interesting  and  difficult  question 
how  this  fluid  rises  to  the  tops  of  such  immense  trees 
as  the  Sequoia  and  the  Eucalyptus,  which  sometimes 
reach  heights  of  500  feet,  and  in  which  the  action  of 
gravity  would  tend  to  produce  outward  pressures 
within  the  roots  of  fifteen  atmospheres.  Suffice  it  to 
say  that  in  some  manner  the  fluids  obtained  from  the 
ground  do  reach  all  parts  of  the  plant,  and  the  water 
copiously  passing  through  the  leaves  evaporates. 
This  is  called  transpiration.  The  carbonic  acid  of  the 
air  entering  the  leaves  during  light  action  in  a  manner 
to  be  described  later,  is  altered  and  combined  with  the 
various  elements  transported  from  the  roots.  Com- 

333 


THE  SUN 

plex  nourishing  compounds  produced  in  the  leaves 
descend  to  all  living  cells  of  the  stem  and  roots,  and 
after  undergoing  further  transformations  even  re- 
ascend  in  spring  to  start  the  new  growth  of  shoots, 
leaves,  and  buds. 

The  various  elements  are  not  available  to  the  plant 
in  all  their  chemical  combinations,  and  in  some  com- 
binations may  even  be  poisonous.  Without  going 
into  details,  it  will  be  interesting  to  note  the  case  of 
nitrogen.  This  element,  found  uncombined  as  a  gas 
in  air,  is  rather  inert  chemically,  and  none  of  the 
higher  plants  seem  able  to  make  use  of  it  in  its  free 
state.  Ammonia,  too,  though  prevalent  as  a  product 
of  decay  in  the  soil,  and  existing  also  in  the  air,  is  not 
nourishing  to  most  of  the  higher  plants.  Nitrites  are 
said  to  be  poisonous  in  moderate  concentration,  al- 
though in  very  dilute  solution  perhaps  useful.  Ni- 
trates, then,  are  to  be  regarded  as  the  principal  nitro- 
gen sources  for  auto  trophic  plants.  In  agriculture 
the  removal  of  crops  withdraws  available  nitrogen 
from  the  soil  faster  than  ordinary  processes  can  pro- 
duce it,  hence  the  use  of  fertilizers  containing  salt- 
peter. But  the  leguminous  plants,  such  as  peas, 
beans,  clover,  alfalfa,  etc.,  are  said  to  be  able  to  use 
free  nitrogen,  and  it  is  customary  to  plow  under  green 
crops  of  such  nature  to  improve  the  soil.  Careful 
researches  have  shown  that  in  reality  certain  micro- 
organisms, often  present  in  the  soil,  cause  the  forma- 
tion of  nodules  on  the  roots  of  these  leguminous 
plants,  and  that  atmospheric  nitrogen  is  only  assimi- 

334 


THE  SUN'S   INFLUENCE  ON   PLANT   LIFE 

lated  when  these  micro-organisms  inhabit  the  nod- 
ules. Different  Leguminosce  require  different  species 
of  micro-organisms  for  a  successful  partnership  of 
this  kind.  The  micro-organisms  combine  the  free 
nitrogen  into  forms  useful  to  the  plant,  and  the  plants 
supply  other  materials,  perhaps  carbohydrates,  for 
the  micro-organisms.  This  is  but  one  of  many  in- 
stances in  which  the  higher  plant  forms  are  proved  to 
depend  upon  the  activities  of  the  lower,  quite  as  much 
as  the  lower  on  the  higher.  Quite  recently  it  has  be- 
come possible  to  purchase  at  large  seed  stores  cultures 
of  the  proper  micro-organisms,  with  instructions  for 
multiplying  them,  so  that  when  sowing  a  field  with 
clover  or  alfalfa,  for  instance,  the  cultures  may  be 
mixed  with  the  seed  so  that  it  may  be  certain  that  the 
crop  will  not  suffer  from  lack  of  nitrogen. 

THE   ASSIMILATION   OF   CARBON   BY   AUTOTROPHIC 
PLANTS 

Many  plants,  among  which  are  corn  and  others  of 
the  most  valuable  food  plants,  will  thrive  in  water 
cultures  as  well  as  in  the  soil,  although  the  supply  of 
carbon  through  their  roots  is  made  impossible.  Hence 
the  source  of  the  carbon  which  is  a  fundamental  ele- 
ment of  all  organic  life  must  be,  in  such  cases,  the  air. 
If  grown  in  darkness,  although  in  all  other  respects 
the  conditions  are  retained  identical,  no  considerable 
gain  of  carbon  occurs  and  the  plants  remain  white, 
for  no  chlorophyll  is  formed.  We  find,  then,  that 
carbon  dioxide  of  the  air  is  taken  in  under  the  influ- 

335 


THE   SUN 

ence  of  light,  and  is  acted  upon  to  form  complex  com- 
pounds with  hydrogen  and  oxygen,  such  as  hexose, 
sugar,  starch,  and  also  the  nitrogenous  carbon  com- 
pounds which  go  to  compose  vegetation.  This  ac- 
tion is  found  to  require  the  green  chlorophyll  bodies 
of  the  living  plant  cells,  and  chlorophyll,  as  we  have 
said,  is  not  produced  without  light.  Oxygen  is  given 
off  in  the  chemical  transformations  and  escapes  from 
the  leaves.  The  process  of  absorbing  carbonic  acid 
and  transforming  it  with  evolution  of  oxygen,  as  just 
described,  is  called  assimilation. 

The  evolution  of  oxygen  may  be  demonstrated  in  a 
simple  way  by  cutting  a  branch  of  the  water  plant 
Elodea  Canadensis  and  placing  it  in  a  tube  of  water 
charged  with  carbonic  acid  gas.  If  retained  in  very 
dim  light  for  some  time  nothing  easily  noteworthy 
occurs,  but  when  well  illuminated  a  stream  of  bub- 
bles escapes  from  the  cut  ends.  By  inverting  a  test 
tube,  previously  filled  with  water,  the  gas  may  be  col- 
lected, and  will  be  found  by  testing  it  with  a  glowing 
coal  to  consist  largely  of  oxygen.  Quantitative  experi- 
ments have  been  made  by  counting  the  bubbles  given 
off  in  this  manner,  and  it  has  thus  been  shown  that 
their  number  is  usually  nearly  proportional  to  the  in- 
tensity of  the  light.  In  darkness  no  oxygen  is  given 
off,  but  carbon  dioxide  is  slowly  evolved  instead.  This 
reverse  process  is  called  respiration. 

As  already  stated,  an  essential  condition  for  carbon 
assimilation  is  the  presence  in  the  plant  of  the  green 
substance  of  chlorophyll.  This  is  found  in  most 


THE   SUN'S   INFLUENCE   ON   PLANT   LIFE 

plants  almost  solely  in  the  leaves,  so  that  these  are 
the  great  organs  of  assimilation.  Chlorophyll  in  alco- 
holic solution  has  a  fine  fluorescence.  It  appears 
green  by  transmitted  light  and  red  by  reflected  light. 
The  spectrum  of  crude  chlorophyll  in  alcoholic  solu- 
tion is  characterized  by  six  absorption  bands.  Three 
are  in  the  violet  and  merge  together  in  strong  chloro- 
phyll solutions.  The  other  three  occur  in  the  green, 
yellow,  and  red  respectively.  By  treating  the  alco- 
holic solution  with  benzine  the  crude  chlorophyll, 
which  is  a  mixture,  will  yield  in  benzine  solution  a 
blue  green  dye  which  seems  to  be  the  more  important 
component.  This  itself  is  complex,  and  contains 
among  other  constituents  one  which  is  called  phyllo- 
porphyrin,  and  differs  only  a  very  little  chemically 
from  the  hsematoporphyrin  of  blood.  But  however 
curious  and  interesting  chlorophyll  may  be,  its  spe- 
cial function,  the  promotion  of  carbon  assimilation, 
does  not  go  on  except  the  chlorophyll  be  in  the  living 
plant  cells.  Artificial  chlorophyll  bearing  cells  will 
not  answer. 

It  has  been  shown  that  for  every  volume  of  carbon 
dioxide  operated  on  by  the  plant,  an  equal  volume  of 
oxygen  is  liberated.  Among  the  principal  products  of 
the  reaction  is  glucose  or  starch.  Such  facts  may 
imply  some  such  actions  as  are  expressed  in  the  fol- 
lowing symbolic  manner. 

6CO2  +  6H2  O  =  C0  H12  O6  (Glucose)  +  6O2; 

or    6C02  +  5H2  O  =  C0  H10  O6  (Starch)  +  6O2. 

33? 


THE  SUN 


Starch  is  readily  demonstrated  as  being  produced  in 
many  plants  during  light  action,  but  plants  of  dif- 
ferent families  vary  greatly  in  the  quantity  of  it  they 
produce.  Indeed,  as  we  shall  see  many  times,  the 
different  plants  behave  so  differently  under  given 
conditions  that  hardly  a  single  general  fact  can  be 
stated,  in  regard  to  which  some  kinds  do  not  exhibit 
exceptions.  As  one  person  is  repelled  by  coaxing  and 

moved  by  argu- 
ed ment,  while  another 
goes  only  as  senti- 
ment dictates,  so 
the  plants  seem  to 
have  their  diverse 
characters,  and  two 
kinds  may  react 
oppositely  to  the 
same  stimuli. 

The  organs  of 
carbon  assimilation 
are  the  leaves,  and 
in  these  the  portals  of  access  are  the  little  open- 
ings called  stomata.  These  exist  in  most  plants  most 
plentifully  on  the  under  surfaces  of  the  leaves,  al- 
though found  in  some  only  on  the  upper  surfaces  and 
in  still  others  on  both.  They  are  very  minute  slit- 
like  orifices,  so  small  that  a  needle  prick  is  a  huge  hole 
in  comparison  with  one  of  them.  A  single  leaf  of  a 
sunflower  may  have  no  less  than  13,000,000  stomata. 
Fig.  66  (after  Schwendener)  gives  a  general  idea  of 

338 


B 


FIG.  66. — STOMATA.     (Schwendener.) 
A,  Cross-section  on  m,  n.      B,  Plan  view 
of  half-stoma  omitting  parts  outside  a, 
b.     C  and  D,  Closed  and  open  stomata. 


THE  SUN'S  INFLUENCE  ON  PLANT  LIFE 

the  form  and  surroundings  of  these  minute  but  nec- 
essary organs  in  the  Amaryllis.  There  are  special  con- 
trivances called  guard  cells  adapted  for  opening  and 
closing  the  stomata.  These  guard  cells,  when  dis- 
tended by  containing  much  liquid,  or  when  shone 
upon  by  strong  light,  cause  the  stomata  to  open  wide, 
thus  promoting  the  assimilation  of  carbon  dioxide, 
and  also  the  transpiration  of  water  vapor,  of  which 
we  shall  speak  later. 

Although  very  numerous,  the  combined  area  of  the 
stomata,  even  when  wide  open,  is  hardly  more  than 
one  per  cent  of  the  area  of  the  leaves,  so  that  it  was 
long  a  mystery  how  so  much  carbon  dioxide  could 
pass  through  them.  This  question  was  solved  by 
Brown  and  Escombe  (1900).  They  found  that  when 
carbon  dioxide  is  admitted  through  an  orifice  to  a 
medium  capable  of  absorbing  the  gas  as  fast  as  re- 
ceived, the  amount  which  diffuses  through  the  open- 
ing decreases  with  the  diameter,  not  with  the  area  of 
the  opening.  This  seeming  paradox  is  explained  by 
supposing  the  velocity  of  flow  to  increase  as  the  open- 
ing decreases,  so  that  a  smaller  hole  accommodates 
the  diffusion  not  merely  from  directly  above,  but  also 
from  the  side  areas  which  were  before  served  by  the 
larger  hole.  The  observers  found  that  their  strange 
new  law  held  for  numerous  openings  as  well  as  for 
single  ones,  provided  the  openings  are  separated  by 
distances  as  great  as  eight  or  ten  times  the  diameter 
of  the  holes.  From  this  it  follows  that  a  surface 
pierced,  like  a  leaf,  with  extremely  numerous  but  very 

339 


THE  SUN 

small  openings  can  admit  the  passage  of  a  gas  by  dif- 
fusion at  almost  as  rapid  a  rate  as  if  the  whole  area  of 
the  surface  were  one  hole.  This  extraordinary  dis- 
covery raises  our  admiration  of  this  excellent  con- 
trivance of  Nature,  whereby  the  whole  area  of  the 
leaves  of  a  plant  is  as  if  available  to  transmit  nour- 
ishment from  the  air,  and  to  permit  the  escape  of 
water  vapor,  although  in  reality  nearly  all  of  this 
area  is  actually  closed,  to  protect  the  delicate  cells 
within. 

The  rate  of  assimilation  of  carbon,  or  what  is  al- 
most proportional  to  it,  the  rate  of  gain  in  dry  weight 
of  a  plant,  depends  on  various  factors.  Of  these  we 
may  mention  first  the  concentration  of  the  carbon 
dioxide  in  the  air.  Although,  according  to  Eber- 
meyer's  estimates,  a  square  mile  of  forest  uses  up  over 
500  tons  of  carbon  dioxide  in  a  year,  so  that  the 
demands  of  the  plant  life  of  the  world  are  really 
enormous,  there  is  nevertheless  an  almost  steady, 
and  everywhere  nearly  uniform,  percentage  of  car- 
bon-dioxide in  the  air — about  three  parts  in  10,000. 
The  steady  drain  of  plant  life  is  to  be  set  over 
against  the  production  of  carbon  dioxide  by  the  res- 
piration of  animals,  the  burning  of  wood  and  coal 
and  other  sources  of  supply,  but  it  is  surprising  that 
the  atmospheric  proportion  remains  so  nearly  uni- 
form as  it  does.  Geologists  are  by  no  means  of  the 
opinion  that  this  proportion  has  always  been  the  same 
as  at  present.  It  is  therefore  of  interest  to  inquire  how 
the  assimilation  varies  with  the  concentration  of  car- 

340 


THE   SUN'S   INFLUENCE   ON   PLANT   LIFE 

bon  dioxide.  There  seems  to  be  some  disagreement 
between  investigators  as  to  the  precise  optimum  con- 
centration, but  all  are  agreed  that  the  rate  of  assim- 
ilation of  carbon  increases  steadily  with  the  concen- 
tration of  carbon  dioxide  up  to  a  concentration  of  at 
least  more  than  ten  fold  the  present.  Under  such  con- 
centrations the  rate  of  assimilation  may  reach  more 
than  twice  its  usual  value.  According  to  some,  the 
increase  of  assimilation  is  even  directly  proportional 
to  the  increased  concentration  of  CO2,  within  these 
limits.  While,  of  course,  this  change  has  no  practical 
interest  while  the  carbon  dioxide  of  the  air  remains 
constant,  yet  it  may  have  been  of  considerable  prac- 
tical importance  to  vegetation  in  past  geological 
epochs  when  the  air  was  more  highly  charged. 

Temperature  is  a  still  more  important  agency  in 
regulating  the  growth  of  plants.  Assimilation  may 
be  recognized  with  some  plants  at  temperatures 
of  several  degrees  below  freezing,  but  practically 
speaking  all  growing  plants  of  the  higher  forms 
must  be  maintained  at  temperatures  between  0°  and 
50°  C.  The  increase  of  the  rate  of  assimilation  for 
most  plants  is  very  rapid  from  0°  up  to  a  temperature 
of  about  35°,  and  at  higher  temperatures  than  this 
there  is  a  still  more  rapid  decrease  in  the  rate.  It  is 
an  interesting  question  whether  the  principal  forms 
of  vegetation  could  flourish  on  any  planet  if  the  mean 
temperature  lay  below  0°,  or  above  50°.  Although  we 
cannot  answer  this  question  absolutely,  still  it  seems 
probable  that  the  answer  must  be  in  the  negative. 

341 


THE  SUN 

At  all  events,  we  see  not  only  how  entirely  our  own 
lives  now  depend  on  the  sun,  but  even  on  that  nice 
balance  between  the  receipt  of  solar  radiation  and  the 
emission  of  terrestrial  radiation,  in  which  even  the 
amounts  of  water  vapor  and  cloudiness  are  important, 
as  stated  in  the  preceding  chapter. 

We  now  take  up  the  dependence  of  carbon  assimi- 
lation on  light,  deferring  the  consideration  of  other 
effects  of  light  on  growth.  Plants  raised  in  darkness 
do  not  become  green.  The  formation  of  chlorophyll 
and  the  assimilation  of  carbon  require  radiation  be- 
tween wave  lengths  0.39//,  and  0.77/*.  Experiments 
on  the  relative  effectiveness  of  rays  of  different  wave 
lengths  are  not  altogether  satisfactory.  They  have 
been  confined  to  a  few  kinds  of  plants,  and  great  dif- 
ficulty is  found  here,  as  in  physics  and  astronomical 
work,  in  separating  a  sufficiently  intense  nearly 
monochromatic  beam  of  light,  and  in  measuring  its 
intensity.  Investigations  were  made  about  thirty 
years  ago  on  the  relative  efficiency  of  the  different 
rays  by  Reinke  and  by  Engelmann.  They  agree  in 
fixing  the  wave  length  of  maximum  effect  in  the  red  at 
about  0.65//,  to  0.70//,,  but  Engelmann  found  a  secon- 
dary maximum  in  the  blue  at  0.48/A,  not  found  by 
Reinke.  Engelmann's  observations  distinguish  be- 
tween the  assimilation  of  the  upper  and  lower  sides 
of  a  leaf  capable  of  such  action,  and  he  finds  the  posi- 
tion of  maximum  effect  shifted  distinctly  towards 
shorter  wave  lengths  for  the  surface  which  receives 
its  illumination  through  the  leaf.  This  result  de- 

342 


THE  SUN'S   INFLUENCE  ON   PLANT  LIFE 

pends,  it  is  thought,  on  the  strong  absorption  by  chlor- 
ophyll for  red  rays,  for  thereby  the  light  which  pene- 
trates the  leaf  is  greatly  weakened  in  its  longer  waves. 
Undoubtedly  the  relative  activity  of  different  wave 
lengths  of  light  in  promoting  the  assimilation  of  car- 
bon is  closely  associated  with  the  absorption  spectrum 
of  chlorophyll,  as  indeed  would  appear  from  Engel- 


0.42 


#50 


#58 


D 


FIG.  67. — PROMOTION  OF  CARBON  ASSIMILATION  BY  LIGHT  (full  curve) 
AND  ABSORPTION  OF  LIGHT  (dotted  curve)  IN  GREEN  LEAVES. 
(Engelmann.) 


mann's  results,  given  in  Fig.  67.  There  is  needed 
much  more  research  in  this  difficult  field.  It  would 
be  greatly  promoted  by  the  introduction  of  means  for 
obtaining  nearly  monochromatic  light  of  well  deter- 
mined and  adequate  intensity,  covering  considerable 
areas  suitable  for  plant  growth  under  otherwise  nat- 
ural conditions. 

24 


THE  SUN 

Kniep  and  Minder 1  have  recently  made  observa- 
tions by  the  bubble  method  with  Elodea  Canadensis 
on  the  assimilation  of  carbon  dioxide  in  lights  of  dif- 
ferent colors.  They  used  sunlight  filtered  by  colored 
solutions  so  as  to  select  red  light  (wave  length  0.62/z, 
to  some  point  in  the  infra-red  not  determined),  green 
light  (wave  length  0.512^  to  0.524/*),  or  blue  light 
(wave  length  0.35yu,  to  0.50//,)  at  pleasure.  In  each  case 
the  light  could  be  reduced  to  a  fixed  intensity  as  meas- 
ured by  a  Rubens  thermopile,  so  that  they  could  in- 
vestigate the  rates  of  assimilation  under  equal  inten- 
sities of  total  radiation  for  each  of  the  three  colors. 
They  found  the  green  light  of  no  effect  in  producing 
assimilation.  It  was  like  no  light  at  all.  The  red 
and  the  blue  they  found  equally  effective.  Hence 
their  experiments  tend  to  support  Engelmann's,  as 
they  indicate  two  wave-length  regions  efficient  to  pro- 
duce assimilation.  We  must  wait  for  more  elaborate 
experiments  before  we  shall  know  just  how  the  effi- 
ciency varies  with  the  wave  length,  and  whether  all 
plants  are  best  promoted  by  the  same  rays.  It  is 
clear,  however,  that  as  the  red  end  of  the  spectrum 
predominates  in  direct  sunlight  at  the  earth's  surface, 
whereas  the  violet  end  greatly  predominates  in  sky 
light,  a  plant  may  be  made  to  assimilate  carbon  pre- 
dominatingly by  red  or  blue  light  according  to 
whether  it  grows  in  direct  sunshine  or  not.  This 
may  offer  a  method  of  evolving  new  plant  forms  as 
we  shall  see  in  the  next  section. 

1  Zeitschrift  fur  Botanik,  vol.  i,  pp.  619-650,  1909. 
344 


THE  SUN'S   INFLUENCE  ON  PLANT  LIFE 

Experiments  have  been  made  on  the  dependence 
assimilation  on  the  total  intensity  of  light  irrespective 
of  wave  length.  Results  very  naturally  differ  for 
plants  of  light-loving  and  shade-loving  habits.  In 
general,  the  rate  of  carbon  assimilation  increases 
nearly  directly  proportionally  to  the  intensity  of  the 
light,  but  this  ratio  of  course  cannot  persist  for  ex- 
tremely high  intensities,  because,  first,  of  injury  to 
the  plant,  and,  second,  of  deficiency  of  other  promot- 
ing elements,  especially  carbon  dioxide. 

ETIOLATION,  OR  EFFECTS  OF  DEFICIENCY  OF  LIGHT 

Plants  grown  in  darkness  or  weak  light  tend  to 
have  long  stem  internodes  and  leaf  stems,  and  small 
white  or  yellow  leaves.  These  and  other  effects  of 
deficiency  of  light  on  plant  growth  are  termed  etiola- 
tion. As  already  stated,  the  higher  plants  do  not  in- 
crease much  in  dry  weight  unless  exposed  to  light,  so 
that  experiments  on  the  effects  of  complete  darkness 
on  growth  are  mainly  restricted  to  such  species  as 
have  large  food  stores  in  their  seeds  or  tubers.  The 
object  served  by  natural  etiolation  is  at  length  to 
bring  the  leaves  of  the  plant  to  suitable  illumination, 
as  is  seen  by  the  tall  tree  stems  and  climbing  vines  in 
closely  growing  forests.  In  experimental  work  this 
result  cannot,  of  course,  be  reached,  but  nevertheless 
the  tendency  is  plainly  shown. 

Although,  as  stated  above,  the  effect  of  darkness 
or  very  weak  illumination  is  to  restrict  the  leaf  area, 
leaves  grown  in  moderate  light  are  larger  and  thinner 

345 


THE  SUN 

than  those  grown  in  full  sun.  This  form  of  etiolation 
is  of  importance  to  the  tobacco  industry,  since  large 
thin  leaves  are  preferred  and  command  much  higher 
prices.  Accordingly,  in  Connecticut  and  Florida 
large  fields  of  tobacco  are  now  grown  shaded  by  tents 
of  open-meshed  cloth  to  promote  this  improvement. 
Other  desirable  results  obtained  by  this  device  con- 
sist in  the  greater  and  more  uniform  humidity  of  the 
air  and  soil  and  the  prevention  of  disastrous  winds 
and  hail. 

With  many  kinds  of  plants  the  buds  will  not  de- 
velop if  the  light  is  too  weak,  and  there  are  besides 
many  other  effects  embraced  by  the  general  term 
etiolation.  Curiously  enough  red  light,  which,  as  we 
have  seen,  is  highly  effective  for  promoting  carbon 
assimilation,  in  many  cases  behaves  like  darkness  in 
respect  to  etiolation.  It  is  thus  possible  to  grow 
plants  under  conditions  favorable  to  their  adequate 
nourishment,  and  at  the  same  time  to  greatly  alter 
their  forms  by  etiolation  effects.  This  interesting 
feature  perhaps  offers  opportunities  for  promoting 
the  evolution  of  desired  forms  in  useful  plants. 

PLANT  GEOGRAPHY 

In  natural  surroundings  there  is  a  very  great  range 
of  light  intensity,  and  with  it  a  great  range  in  temper- 
ature and  moisture.  These  circumstances  produce 
very  marked  effects  on  plant  life.  In  the  tropics 
abound  regions  of  great  rainfall,  from  100  to  500  cen- 
timeters (40  to  200  inches)  annually,  with  the  aver- 

346 


THE  SUN'S   INFLUENCE   ON   PLANT   LIFE 

age  cloudiness  as  high  as  fifty  to  sixty  per  cent.  The 
mean  temperature  being  also  high,  25°  to  28°  C., 
there  results  a  High  atmospheric  humidity.  Such  re- 
gions are  the  home  of  the  tropical  rain  forest,  which, 
viewed  from  a  ship  presents  a  noticeably  irregular 
skyline  filled  with  every  shade  of  green,  but  tending 
toward  the  more  sombre  hues.  Flowering  trees  are 
occasionally  conspicuous.  The  interior  of  such  a 
forest  teems  with  a  varied  mass  of  vegetation  from 
the  ground  to  the  top  of  the  highest  trees.  Vines  and 
rugged  ferns  abound,  so  that  the  traveler's  way  is 
almost  impassable.  Fruits  and  flowers  are  plentiful. 
Parasitic  and  saprophytic  plants  revel  among  the 
luxurious  surroundings.  On  viewing  a  tall  tree  one 
can  scarcely  distinguish  which  is  its  own  foliage  and 
which  that  of  the  dependent  vines  and  parasites  that 
load  its  trunk  and  limbs  almost  to  breaking. 

Sharply  contrasting  with  such  scenes  as  this  are 
the  sub-tropical  deserts  like  the  African  Sahara. 
Here  also  the  temperature  is  high,  but  variable,  rang- 
ing perhaps  20°  C.  in  a  single  day.  Rainfall  may  be 
as  slight  as  5  centimeters  annually,  but  more  often 
reaches  10  to  20  centimeters.  The  scanty  vegetation 
is  provided  with  extraordinary  contrivances  to  re- 
duce as  far  as  possible  the  loss  of  water  by  transpira- 
tion. Leaves  are  small,  thick,  glossy  and  waxy,  their 
stomata  protected  heavily.  Thorns  abound.  The 
roots  run  very  deep  so  that  even  at  one  or  two  meters 
in  depth  they  have  hardly  diminished  at  all  in  size. 
As  a  rule  only  small  plants  and  shrubs  are  found. 

34:7 


THE  SUN 

Some  varieties  have  special  reservoirs  for  the  storage 
of  water. 

The  periods  of  rest  are  not  conspicuous  in  the  trop- 
ical vegetation,  for  they  are  not  governed  by  temper- 
ature, though  perhaps  often  by  rainfall.  Tropical  rest 
periods  are  frequently  localized  to  single  trees  or  parts 
of  trees;  but  in  the  temperate  and  arctic  zones  there 
occurs  in  winter  manifestly  a  general  cessation  of 
growth.  Not  all  temperate  and  arctic  trees  cast 
their  leaves,  but  they  generally  rest  from  the  growth 
of  shoots  during  the  cold  months.  Askenasy  has  in- 
vestigated these  matters  at  Heidelberg  for  the  gean 
tree,  which  may  serve  as  a  type  for  other  broad- 
leaved  trees.  The  season  of  activity  lasts  from  about 
mid-April  to  mid-October.  It  comprises  the  period 
of  growth  of  foliage,  April-May,  during  which  next 
season's  foliage  buds  appear;  then,  May-September, 
follows  the  period  of  assimilation  during  which  stems 
and  roots  enlarge  and  the  next  season's  flower  buds 
are  formed;  then  comes  the  period  of  decline  ending 
in  the  fall  of  leaves.  During  the  summer  the  growth 
of  next  season's  buds  is  slow,  and  ceases  altogether 
from  October  to  early  Februarjr.  Then  a  growth 
begins  and  becomes  more  and  more  rapid.  Although 
a  warm  March  greatly  accelerates  the  development, 
a  warm  October  cannot  start  growth.  From  the  end 
of  November,  development  may  be  forced  by  hot- 
house conditions.  During  the  rest  period  chemical 
changes  of  the  reserve  material  go  on,  and  it  is  indeed 
transported  between  different  organs  of  the  tree, 

348 


THE  SUN'S   INFLUENCE   ON  PLANT  LIFE 

Temperate  forests  contrast  with  the  tropical  rain 
forest  described  above  in  the  relative  absence  of  vines, 
parasitic  vegetation,  and  undergrowth,  although  in 
moist  regions  herbs  and  shrubs  are  not  lacking.  The 
evergreen  conifers  are  more  and  more  in  evidence  at 
higher  latitudes,  but  these  become  dwarfed  toward 
the  arctic  zones.  The  growth  period  of  arctic  flora 
is  limited  to  about  two  months,  but  is  favored  by  the 
fact  that  the  sun  is  then  continually  above  the  hori- 
zon. All  varieties  start  their  growing  almost  simul- 
taneously, and  reach  their  flowering  stage  almost 
together,  within  a  couple  of  weeks.  Although  the 
mean  temperature  of  the  air  during  the  growing  pe- 
riod may  be  5°  C.  or  more,  the  soil  is  frozen  almost  to 
the  surface. 

Wiesner  has  made  extensive  photographic  re- 
searches to  determine  the  light  requirements  of 
plants.  He  employs  a  modification  of  the  method  of 
Bunsen  and  Roscoe.  A  normal  photographic  paper 
is  prepared  by  soaking  in  three  per  cent  common  salt 
solution,  drying  in  darkness,  soaking  five  minutes  in 
twelve  per  cent  silver  nitrate  solution,  and  drying 
again  in  darkress.  A  normal  tone  or  grade  of  dark- 
ening is  prepared  by  coating  a  paper  with  a  mixture 
of  one  part  lampblack  in  1,000  parts  zinc  oxide.  When 
the  photographic  paper  reaches  this  shade  by  expos- 
ure to  light  for  one  second,  the  light  is  said  to  be  of 
unit  intensity  of  the  Bunsen-Roscoe  scale.  Those  au- 
thors showed  that  for  equal  blackening  of  the  photo- 
graphic paper  the  intensity  of  the  light,  between'  wide 

349 


THE  SUN 

limits,  is  inversely  as  the  time  required.  Hence  if  n 
seconds  are  required  to  produce  normal  tone,  the 
light  intensity  is  1  /  n  Bunsen-Roscoe  unit.  To 
avoid  inconveniently  long  exposures  in  deeply  shaded 
places,  and  to  allow  sufficient  time  for  accurate  re- 
sults in  strong  light,  Wiesner  introduced  a  gradation 
of  shades,  forming  a  kind  of  tone  scale,  which  he 
standardized  in  terms  of  the  normal  tone. 

By  such  procedures  Wiesner  has  measured  the 
light  action  due  to  direct  sunlight,  and  to  diffused 
light  at  Buitenzorg  (Java),  Cairo  (Egypt),  Vienna 
(Austria),  several  stations  in  Norway,  and  Advent 
Bay  (Spitzbergen).  His  measures  were  made  on' days 
varying  in  brightness  from  cloudlessness  to  rain  and 
snow.  The  Vienna  measurements  extend  for  several 
years.  He  has  made  observations  in  the  open,  in 
leafy  tree  crowns,  and  under  the  shadow  of  thick  for- 
ests. It  is  not  possible  to  give  here  any  adequate 
summary  of  this  very  extensive  work,  but  the  reader 
may  consult  the  original  articles  of  Wiesner.1 

Some  of  Wiesner's  results  are  as  follows:  The  max- 
imum total  illumination  at  Vienna  was  1.50  B-R 
units;  at  Buitenzorg,  1.61.  At  Vienna  the  mean 
midday  value  ranges  from  0.1  B-R  unit  in  January  to 
0.96  in  July.  At  Buitenzorg  in  December  and  Janu- 
ary the  midday  values  range  from  0.65  to  0.85.  Rain 
or  snow  diminishes  the  light  total  to  one-tenth  or  less 

1  Especially  in  Sitzungsberichte  Wien.  Akad.  Math.  Naturw.  Kl., 
Bd.  102,  I,  1893;  104,  I,  1895;  109,  I,  1900;  113, 1,  1904.  Also 
Denkschriften  of  the  same  Academy,  Bd.  64, 1897;  67,  1899. 

350 


THE  SUN'S   INFLUENCE  ON   PLANT  LIFE 


of  its  normal  value.  At  Vienna  the  ratio  of  direct 
sunlight  to  diffused  skylight  action  is  very  variable, 
but  an  average  value  for  several  hours  near  midday  is 
about  unity.  On  half  cloudy  days  the  total  light 
action  is  almost  as  strong  as  on  cloudless  days.  On 
completely  cloudy,  but  not  stormy,  days  the  total 
light  action  is  reduced  from  three-  to  five-fold. 

Taking  the  total  direct  and  diffused  light  action  in 
the  open  as  the  basis  of  reckoning,  Wiesner  compares 
with  it  the  light  action  found  in  the  crowns  of  trees 
and  elsewhere.  Calling  the  first  value  I,  the  second 
ij  he  calls  the  ratio  (i/I  =  L)  the  relative  photic  ration. 
When  the  leaves  are  beginning  to  form  in  spring,  be- 
fore they  get  large  enough  to  cast  deep  shadows,  the 
values  of  L  within  tree  crowns  and  under  trees  are 
not  greatly  less  than  unity.  But  later  in  the  summer, 
when  leaves  are  full  grown,  and  next  season's  leaf 
buds  are  forming,  these  ratios  become  much  smaller. 
For  instance  for  the  white  birch  (Betula  alba), 
Wiesner  finds: 


Observed  values  of  I 

Date 

L  f         Day's        ) 
\     Minimum    j 

Total  daylight 

In  tree  crown 

• 

1 

April  16  

0.834 

0.333 

275 

1 

May     1  

0.875 

0.219 

4 

1 

May  14  

1.122 

0.142 

8 

May  29  

1.200 

0.109 

ll 

351 


THE   SUN 

This  rapid  increase  of  the  shading  action  of  forest 
trees,  as  they  develop  their  leaves,  determines  the 
nature  and  habit  of  the  underbrush.  Generally  the 
leaves  of  the  underbrush  present  a  scattered,  or  flat 
array,  so  as  not  to  shade  one  another.  Often  the 
undergrowth  has  the  habit  of  rapid  development  of 
leaves  and  blossoms  in  early  spring,  before  the  over- 
growth is  fully  leaved. 

In  arctic  regions  the  vegetation,  almost  without 
exception,  requires  practically  all  the  available  light. 
This  depends,  no  doubt,  on  the  coldness  and  short- 
ness of  the  season  of  growth.  Values  of  L  much 
below  unity  seem  to  be  insufficient  for  arctic  plants. 
This  may  explain  the  absence  of  tree  forms  there. 
Whereas  in  the  tropics,  and  even  in  temperate  zones, 
most  plants  have  means  for  reducing  the  light  action 
on  their  leaves,  no  such  contrivances  are  common  in 
the  frigid  zones. 

The  range  of  light  requirements  is  indicated  by  the 
following  values  of  the  relative  photic  ration  and 
total  light  action  within  the  crowns  of  trees  in  full 
leaf.  The  terms  (Max.)  and  (Min.)  refer  to  the  max- 
imum and  minimum  daily  values  of  the  quantities 
concerned. 

Among  underbrush  growing  in  a  shade  so  deep  that 

L  =  —  he  found  beeches,  maples,  and    other  well 

oo 

growing  saplings.  Grasses  in  the  temperate  zones 
were  found,  although  not  blooming,  when  L  =  — -. 

uU 

352 


THE  SUN'S   INFLUENCE   ON   PLANT   LIFE 


Common  name 

L  (Min.) 

I  (Max.) 

Remarks 

Boxwood  

J 

0.012 

108 

Beech  

1 

85 

0.015 

Isolated  tree 

Maple  

1 
43 

0.030 

Isolated  tree 

White  poplar.        

1 

0  086 

Isolated  tree 

15 

Pine 

1 

0  118 

Enclosed  tree 

11 

White  birch  

1 

0.144 

Enclosed  tree 

9 

Ash  

1 
5.8 

0.224 

Enclosed  tree 

Larch 

1 

0  260 

Isolated  tree 

5 

Blackthorn  

_J_ 

0.722 

Blooming  but  not  leaved 

1.3 

Some  tropical   grasses   survive  L  ==  -  — .     Lichens 

luu 

were  found   in    the   tropics  which  had  the  photic 

ration  only  L  =  — - -.     Many  forms  of  tropical  or- 
ZoO 

chids,  epiphytes,  and  other  shade-loving  plants,  were 

found  to  thrive  under  photic  rations  from  —  to  — . 

IU       ou 

We  cannot  dwell  longer  on  the  interesting  work  of 
Wiesner.  From  it  we  see  how  unnecessary  it  is,  for 
many  forms  of  vegetation,  that  the  light  should  be  of 
the  full  intensity  which  is  now  available  in  the  open. 
Indeed,  Wiesner  remarks  that  in  experiments  made 
by  rotating  plants,  so  as  to  get  equalized  illumination, 
the  buds  will  develop  and  leaves  be  fully  grown  under 
illuminations  far  below  the  minima  observed  under 

353 


THE   SUN 

natural  conditions.  In  the  natural  state  the  well- 
illuminated  buds  grow  at  the  expense  of  their  less 
favored  neighbors,  and  as  their  leaves  expand  they 
tend  still  further  to  suppress  the  undeveloped  buds. 
In  view  of  all  this,  and  in  view  of  the  hypothesis  (B) 
advanced  in  Chapters  VI  and  VII,  which  treated  of  a 
more  uniform  illumination  assumed  to  be  formerly 
prevailing,  it  is  interesting  to  speculate  whether  the 
great  vegetation  of  the  Carboniferous  era  was  not 
produced  under  a  far  more  feeble  illumination  than 
that  which  now  prevails. 

Considering  the  present  lack  of  exact  experiments 
on  the  efficiency  of  different  wave  lengths  of  light  to 
promote  plant  growth,  the  photographic  experiments 
of  Wiesner  are  perhaps  all  that  are  yet  demanded. 
But  we  can  easily  see  the  advantage  which  would  re- 
sult to  plant  physiology  if  such  an  instrument  as  the 
the  spectrobolometer  could  be  employed  in  skilled 
hands  to  determine  the  relations  of  wave  length  and 
intensity  of  light  to  carbon  assimilation  and  etiola- 
tion, for  numerous  plant  forms. 

HELIOTROPISM 

It  is  well  known  that  different  plants  vary  as  re- 
gards the  angles  which  their  organs  present  to  the 
direction  of  strongest  light.  A  nasturtium,  for  in- 
stance, if  principally  illuminated  from  one  direction, 
will  expose  almost  every  leaf  and  flower  with  its  face 
broadside  toward  the  light.  Plants  within  a  room 
bend  toward  the  window.  Some  species  which  live 

354 


THE   SUN'S   INFLUENCE   ON   PLANT   LIFE 


in  dry  and  cloudless  regions  present  their  leaves  edge- 
wise to  the  strongest  illumination.  Such  adaptations 
as  those  we  have  mentioned,  and  others,  are  em- 
braced under  the  term  heliotropism.  Different  plant 
organs  differ  in  respect  to  the  matter,  so  that  botan- 
ists distinguish  organs  which  are  orthotropic,  and 
those  which  are  plagiotropic,  according  as  they  tend 
to  lie  in  the  direction  of  the  principal  light  or  at  some 
other  angle  with  respect  to  it.  Also  orthotropic 
organs  may  grow  in  a  positive  heliotropic  manner, 
i.  e.  towards  the  source  of  light,  or  the  contrary. 
Roots  are  usually  negatively  and  stems  positively 
orthotropic,  while  leaves  may  be  regarded  as  plagio- 
tropic. 

It  was  supposed  by  De  Candolle  (1832)  that  helio- 
tropism was  a  simple  consequence  of  different  rates 
of  growth  between  strongly  and  weakly  illuminated 
parts  of  an  organ.  It  had  been  found  (as  already 
stated  under  etiolation)  that  stems  grown  in  darkness 
exceeded  in  length  those  grown  in  light.  Further- 
more, it  has  been  shown  that  plants  increase  in  stat- 
ure faster  by  night  than  by  day.  See,  for  instance, 
the  following  measurements  by  Kraus  on  the  growth 
of  a  species  of  bamboo  at  Buitenzorg,  Java,  in 
twelve-hour  intervals : 


Date 

Dec.  4 

Dec.  5 

Dec.  6 

Dec.  7 

Dec.  8 

Growth  by  day  .... 

10.5  cm. 

4.5  cm. 

8cm. 

8.5  cm. 

12  cm. 

Growth  by  night  .  .  . 

16cm. 

15  cm. 

16cm. 

12.5cm.- 

355 


THE  SUN 

On  such  grounds  De  Candolle  assumed  that  helio- 
tropic  curvature  was  simply  the  effect  of  the  retard- 
ing influence  of  light  on  the  growth  of  that  side  of  the 
stem  most  strongly  illuminated.  This  simple  ex- 
planation may  have  some  justification,  but  it  is  not 
adequate  to  explain  the  facts.  For  plant  organs 
which  curve  away  from  the  light  also  grow  faster  in 
the  dark.  Furthermore,  the  same  organ  may  react 
either  positively  or  negatively  or  not  at  all  according 
to  the  intensity  of  the  light,  as  shown  by  the  experi- 
ments of  Oltmans.  This  author  is  of  the  opinion 
that  the  best  intensity  of  illumination  for  the  general 
welfare  of  the  organism  is  that  at  which  it  exhibits  no 
heliotropic  curvature.  Direct  sunlight  is  too  bright 
to  induce  heliotropic  curvature  in  most  plants,  hence 
they  do  not  as  a  rule  turn  their  leaves  from  east  to 
west  with  the  progress  of  the  sun,  although  in  the 
case  of  the  sunflower  this  occurs  with  the  blossoms. 

It  seems  that  illumination  acts  rather  as  a  stimulus 
than  as  a  force  in  producing  heliotropism,  for  the 
effect  may  be  produced  by  brief  light  action  and  the 
actual  curvature  take  place  in  the  appropriate  direc- 
tion after  the  light  has  been  withdrawn.  Further- 
more, the  reaction  does  not  necessarily  occur  where 
the  light  is  applied,  but  the  stimulus  may  be  trans- 
mitted some  distance  from  the  sensitive  recipient 
organ  to  the  position  where  curvature  takes  place, 
although  the  part  of  the  organ  where  it  becomes 
curved  is  shielded  entirely  from  the  action  of  the 
light. 

356 


THE  SUN'S   INFLUENCE  ON  PLANT  LIFE 

Heliotropism  is  without  doubt  of  great  value  to 
plants  in  enabling  them  to  adjust  their  leaves  most 
advantageously  to  increase  or  reduce  the  illumina- 
tion in  which  they  find  themselves.  It  is  especially 
valuable  to  many  compound  leaved  plants  subjected 
to  the  powerful  heating  effects  of  the  direct  rays  of 
the  unclouded  sun.  They  open  out  their  leaves  in 
the  early  morning  or  during  cloudy  weather,  but  tilt 
them  up  edgewise  in  the  hot  sun,  thus  reducing  the 
effective  area  for  heating.  'Such  plants,  though  of 
large  leaf  area,  may  thrive  in  the  driest  regions. 
Other  plants  have  their  leaves  permanently  set  at 
such  an  angle  as  to  receive  a  minimum  of  direct  sun- 
light. 

On  the  other  hand  many  plants  growing  in  corn- 
par  atively  weak  light,  and  some  sun-loving  plants  in 
the  open,  turn  the  broad  sides  of  their  leaves  toward 
the  strongest  light.  The  negative  heliotropism  of 
the  roots  of  plants  is  of  advantage,  for  by  it  they  may 
be  saved  from  growing  out  of  the  soil. 

PLANTS  AS  ENERGY  ACCUMULATORS 

The  energy  now  available  in  coal  and  oil  was  for- 
tunately preserved  for  our  use  in  the  decomposed  veg- 
etation of  former  ages.  Extraordinary  luxuriance  of 
vegetation  is  thought  to  have  prevailed  in  those 
ancient  times,  and  we  now  use  the  accumulated  en- 
ergy of  solar  rays  emitted  long  before  the  existence 
of  man.  Attempts  to  employ  solar  energy  by  arti- 
ficial engines  will  be  referred  to  in  the  next  chapter, 

357 


THE  SUN 

but  none  of  them  can  as  yet  be  compared  in  econom- 
ical success  with  the  natural  process  of  storage  always 
occurring  in  vegetable  growth.  Artificial  processes 
are  for  the  moment  far  more  efficient,  but  not  in  pro- 
portion to  their  great  cost,  and  none  of  them,  like  the 
natural  process,  stores  permanently  the  energy  re- 
ceived. Most  solar  engines  transform  solar  radia- 
tion immediately  into  heat,  and  this  is  gradually  lost. 
Growth  transforms  solar  radiation  immediately  into 
chemical  energy,  and  this  may  be  indefinitely  pre- 
served. 

Various  estimates  have  been  made  of  the  efficiency 
of  plants  as  transformers  of  energy.  Pfeffer  (1871) 
computed  from  Boussingault's  work  that  a  square 
meter  of  Nerium  leaf  surface  formed  starch  at  the 
rate  of  0.000535  grams  per  second.  Assuming  the 
product  formed  to  have  a  heat  of  combustion  of  4,100 
calories  per  gram,  he  found  2.2  calories  per  square 
meter  per  second  to  be  the  amount  of  energy  con- 
served. The  amount  of  energy  received  from  the  sun 
would  depend  on  the  time  of  day,  inclinations  of  the 
leaves,  moisture  of  the  air,  etc.,  but  might  be  esti- 
mated at  about  150  calories  per  square  meter  per  sec- 
ond in  ordinary  conditions  near  sea-level.  This 
would  give  an  efficiency  of  about  1.5  per  cent. 

Brown,  in  his  Bakerian  Lecture  (see  Nature,  vol.  Ixxi, 
p.  522),  summarized  some  careful  experiments  on  the 
efficiency  of  the  sunflower.  He  made  estimates  of 
the  temperatures  of  leaf  surfaces  and  of  their  thermal 
emissivity.  The  latter  in  still  air  was  about  0.015 

358 


THE  SUN'S   INFLUENCE  ON   PLANT  LIFE 

calories  per  square  centimeter  of  leaf  surface  for  1°  C. 
difference  of  temperature  from  the  surroundings. 
Leaves  evolve  carbon  dioxide  in  darkness  in  their  or- 
dinary process  of  respiration.  For  a  sunflower  leaf 
respiring  0.7  cubic  centimeters  of  carbon  dioxide  per 
100  square  centimeters  per  hour,  the  respiration 
causes  a  rise  of  temperature  of  the  leaf  in  still  air  of 
0°.019  C.  above  its  surroundings.  This  effect  is 
therefore  practically  negligible.  Not  so  the  effect 
of  transpiration  and  evaporation  of  water,  especially 
in  windy  surroundings,  for  this  may  alter  the  temper- 
ature of  leaves  by  several  degrees.  The  absorption 
coefficients  of  leaves  of  various  plants  in  ordinary 
sunlight  were  determined.  These  range  from  sixty- 
five  to  seventy-eight  per  cent,  and  for  the  sunflower 
leaf  was  found  to  be  68.6  pdr  cent.  Such  values 
would  probably  differ  according  to  the  quality  of  the 
light.  The  rate  of  absorption  of  carbon  dioxide  by 
the  plants  was  measured.  Air  was  drawn  through 
the  glazed  case  containing  the  leaf  specimen  and  the 
carbon  dioxide  contents  of  the  air  after  passage  was 
compared  with  that  of  air  unaffected  by  the  plant. 
Various  concentrations  of  carbon  dioxide  were  experi- 
mented upon,  and  it  appeared  that  up  to  concentra- 
tions six  times  the  normal,  the  rate  of  assimilation 
was  proportional  to  the  concentration  of  carbon  diox- 
ide in  the  air.  The  material  formed  by  the  plant  was 
assumed  to  be  hexose,  whose  heat  of  combustion  is 
3,760  calories  per  gram.  The  rate  of  assimilation 
seemed  to  be  independent  of  the  intensity  of  the 
25  359 


THE  SUN 

light,  until  this  was  reduced  as  low  as  0.04  calories 
per  square  centimeter  per  minute,  or  say  --  of  or- 
dinary sunlight.  Hence  the  efficiency  of  the  plant 
appeared  much  higher  under  weak  illumination.  In 
some  cases  the  efficiency  found  was  as  high  as  five  per 
cent,  but  not  often  above  1.7  per  cent. 

Two  numerical  illustrations  will  show  the  charac- 
ter of  the  results.  Both  deal  with  the  sunflower  leaf 
(Helianthus  annuus).  In  the  first  case  the  activity 
of  the  leaf  did  not  suffice  to  expend  all  the  solar  energy 
it  absorbed  and  the  leaf  was  above  the  temperature  of 
the  surroundings.  In  the  second  case,  owing  to  the 
high  temperature,  the  fully  opened  stomata,  and  the 
low  humidity  prevailing,  there  was  rapid  transpira- 
tion and  the  contrary  state  existed.  The  numbers 
given  in  the  first  part  of  Table  XXIX  apply  to  the 
energy  reaction  per  square  centimeter  of  leaf  surface 
per  minute.  In  the  latter  part  of  the  table  is  given 
the  disposal  of  the  leaf  in  percentages  of  the  solar 
energy  received,  plus  the  heat  energy  received  from 
the  surroundings. 

/  It  appears  from  such  investigations  as  have  been 
made  that  plants  may  store  up  as  chemical  energy  in 
round  numbers  one  or  two  per  cent  of  the  energy  of 
solar  radiation  which  shines  upon  their  leaves.  This 
may  seem  a  very  small  efficiency,  but  on  its  results 
accumulated  through  former  ages  have  depended  the 
\  great  manufacturing  achievements  and  the  comfort- 
able whiter  warmth  of  our  dwellings  for  many  years. 

360 


THE  SUN'S   INFLUENCE  ON  PLANT  LIFE 


TABLE  XXIX. — Economy  of  Helianlhus  annuus 


Case  A 

CaseB 

Total  solar  radiation  received  

0.2569cal. 

0  2746  cal. 

Amount  absorbed  

0.1762 

0.1884 

Amount    of   energy   used    vaporizing 
water 

0  1243 

0  3668 

Amount  of  energy  used  in  photo-syn- 
thesis 

0  0017 

0  0033 

Amount  of  energy  lost  by  cooling  
Velocity  of  wind  in  meters  per  minute.  . 
Temperature  of  leaf  above  surround- 
ings 

+0.0502 

428 

+0°.43C. 

-0.1817 
200 

-1°  84  C. 

Energy  used  in  photo-synthesis  
Energy  used  in  transpiration  

per  cent. 
0.66 
48.39 

per  cent. 
0.72 
80  38 

Solar  energy  transmitted  by  leaf 

31  40 

18  90 

Heat  energy  lost  to  surroundings  

19.55 

In  the  combination  of  water  power  and  electricity  we 
seem  now  to  be  passing  in  a  measure  away  from  the 
dependence  on  coal  and  steam,  but  there  is  little  ques- 
tion that  both  coal  and  oil  will  long  remain  in  exten- 
sive use  to  remind  us  of  our  dependence  on  the 
growth  of  ancient  vegetation  and  its  transformation 
of  solar  radiation  into  chemical  energy. 


CHAPTER   IX 

UTILIZING    SOLAR   ENERGY 

Experiments  with  Burning  Mirrors. — The  "Hot-box"  Principle. — • 
Mouchot,  Pifre,  and  Ericsson. — Eneas'  Solar  Engines. — Proper- 
ties of  Glass. — Solar  Heaters  and  Reservoirs. — Low  Temperature 
Solar  Engines. — Solar  Cooking  Appliances. — Solar  Metallurgy. — • 
Resume.  —  Quantity  of  Solar  Energy  Available.  —  Thermo- 
dynamic  Efficiency. — Reflecting  Powers  of  Mirror  Surfaces. 

AT  present  the  manufacturing  and  commerce  of  the 
world  is  mainly  carried  on  by  aid  of  coal  or  internal 
combustion  engines,  which  derive  their  fuel  from  the 
decomposed  products  of  prehistoric  masses  of  vege- 
tation in  which  were  stored  a  small  fraction  of  the 
solar  energy  of  those  bygone  times.  The  modern 
great  development  of  water  power,  electrically  util- 
ized, also  depends  on  the  sun;  for  by  solar  heating 
water  is  evaporated  from  oceans,  lakes,  rivers,  and  the 
soil,  is  transmitted  inland  and  precipitated  by  the 
atmospheric  circulation  which  the  sun's  heat  main- 
tains, and  comes  in  use  when  it  flows  down  in  the 
rivers.  Another  immense  source  of  water  power,  not 
as  yet  much  utilized,  resides  in  the  ocean  waves  and 
tides,  which  also  depend  in  a  high  degree  upon  the 
sun.  It  is  not  necessary  to  discuss  further  these  well- 
known  sources  of  power,  and  we  shall  pass  to  the  vari- 
ous means  which  have  been  proposed  for  using  the 
energy  of  the  solar  rays  more  directly. 

362 


UTILIZING   SOLAR  ENERGY 

EXPERIMENTS  WITH  BURNING  MIRRORS 

It  is  said  that  during  a  siege  of  Syracuse  in  the  year 
214  B.  c.  the  renowned  philosopher  Archimedes 
burned  or  scattered  the  Roman  fleet  under  Marcellus 
by  concentrating  sun  rays  upon  the  ships  by  means  of 
mirrors  erected  on  the  shore.  Whatever  may  be  the 
truth  of  this  story,  which  has  been  doubted,  such  means 
of  warfare  are  not  likely  to  be  revived  in  our  day. 

Buffon,  the  French  naturalist  (1707-1788),  tested 
the  possibility  of  the  circumstance  just  described.  In 
1747  he  made  many  experiments  with  a  burning  mir- 
ror constructed  by  mounting  360  plane  glass  mirrors, 
each  16  X  22  centimeters,  on  a  frame  in  such  a  man- 
ner that  each  could  be  adjusted  separately,  so  that 
all  could  concentrate  their  reflected  rays  to  a  focus  at 
any  desired  distance.  Corresponding  to  the  angular 
diameter  of  the  sun,  the  focus  was  about  44  centi- 
meters in  diameter  at  50  meters,  and  proportionately 
less  at  shorter  focal  distances.  He  found  it  possible 
to  set  fire  to  wood  at  68  meters.  With  45  mirrors  he 
melted  3  kilograms  of  tin  in  a  pot,  at  6.5  meters,  and 
with  117  mirrors  melted  silver  at  the  same  distance. 
By  these  experiments  he  showed  the  possibility  of  the 
feat  of  war  attributed  to  Archimedes. 

In  1755  Hoesen,  a  mechanician  of  Dresden,  began 
to  build  up  mirrors  of  paraboloidal  curvature.  One 
of  these  was  over  3  meters  in  diameter,  and  so  well 
made  as  to  concentrate  the  sun's  rays  to  a  focus  1.3 
centimeters  in  diameter.  With  one  of  Hoesen's  'mir- 

363 


THE  SUN 

rors  of  half  this  diameter  Wolf  reduced  many  metallic 
ores,  and  melted  coins  almost  instantly. 

THE  " HOT-BOX"  PRINCIPLE 
De  Saussure  (1740-1799),  the  Swiss  naturalist, 
made  five  half  cubes  of  glass  of  such  sizes  as  to  go  one 
within  the  other  with  some  air  space  between.  These 
rested  inverted  on  a  blackened  table  non-conductive 
of  heat.  Thermometers  were  placed  between  the 
vessels  and  in  the  air  outside.  The  one  between  the 
fourth  and  fifth  vessels  showed  the  highest  tempera- 
ture, 87.5°  C.  In  later  experiments  with  glass-covered 
vessels  he  protected  the  sides  and  back  of  the  vessel 
from  cooling  by  wrapping  it  with  non-conducting 
material.  When  the  vessel  was  exposed  to  the  sun 
perpendicularly  he  observed  on  one  occasion  a  tem- 
perature of  110°  C.  within.  In  one  experiment  he 
heated  the  surrounding  medium,  keeping  its  temper- 
ature just  below  the  inside  temperature,  and  thereby 
practically  prevented  loss  of  heat,  except  through 
the  front.  In  this  manner  he  obtained  a  temperature 
within  of  160°  C.  His  experiments  convinced  him 
that  two,  or  at  most  three,  sheets  of  glass  over  such  a 
hot  box  are  better  than  more.  He  made  some  essays 
at  cooking  with  such  devices. 

Sir  John  Herschel  describes  the  following  experi- 
ments made  during  his  sojourn  at  the  Cape  of  Good 
Hope,  1834-1838.1  

1  "  Results  of  Astronomical  Observations  ...  at  the  Cape  of 
Good  Hope,"  etc.,  by  Sir  John  F.  W.  Herschel,  Bart.,  published  1847. 
Appendix  C. 

364 


UTILIZING  SOLAR  ENERGY 

"  (439)  When,  the  heat  communicated  from  the  sun 
is  confined  and  prevented  from  escape,  and  so  forced 
to  accumulate,  very  high  temperatures  are  attained. 
Thus,  in  a  small  mahogany  box  blackened  inside, 
covered  with  window  glass  fitted  to  size,  but  without 
putty,  and  simply  exposed  perpendicularly  to  the 
sun's  rays,  an  inclosed  thermometer  marked,  on  Nov. 
23, 1837, 149°  F. ;  on  Nov.  24, 146°,  150°,  152°,  etc.,  etc. 
When  sand  was  heaped  round  the  box,  to  cut  off  the 
contact  of  the  cold  air,  the  temperature  rose  on  Dec. 
3,  1837,  to  177°.  And  when  the  same  box,  with  its 
enclosed  thermometer,  was  established  under  an 
external  frame  of  wood  well  sanded  up  at  the  sides, 
and  protected  by  a  sheet  of  window  glass  (in  addition 
to  that  of  the  box  within),  the  temperatures  attained 
on  Dec.  3,  1837,  were  at  Ih  30m  P.M.  (Appar.  T.) 
207.0°;  at  Ih  50m,  217.5°;  and  at  2h  44  m,  218°, 
and  that  with  a  steady  breeze  sweeping  over  the  point 
of  exposure.  Again  on  Dec.  5,  under  a  similar  form 
of  exposure,  temperatures  were  observed  at  Oh  19m, 
of  224°;  Oh  29m,  230°;  at  Ih  15m,  239°;  at  Ih  57m, 
248°;  and  at  2h  57m,  240.5°.  As  those  temperatures 
far  surpass  that  of  boiling  water,  some  amusing  ex- 
periments were  made  by  exposing  eggs,  fruit,  meat, 
etc.,  in  the  same  manner  (Dec.  21,  1837,  et  seq.), 
all  of  which,  after  a  moderate  length  of  exposure, 
were  found  perfectly  cooked — the  eggs  being  ren- 
dered hard  and  powdery  to  the  center;  and  on  one 
occasion  a  very  respectable  stew  of  meat  and  vege- 
tables was  prepared,  and  eaten  with  no  small  relish 

365 


THE  SUN 

by  the  entertained  bystanders.  I  doubt  not,  that 
multiplying  the  inclosing  vessels,  constructing  them 
of  blackened  copper  inside,  insulating  them  from  con- 
tact with  each  other  by  charcoal  supports,  surround- 
ing the  exterior  one  with  cotton,  and  burying  it  so 
surrounded  in  dry  sand,  a  temperature  approaching 
ignition  might  readily  be  commanded  without  the 
use  of  lenses." 

MOUCHOT,    PlFRE,    AND    ERICSSON 

August  Mouchot,  of  Tours,  France,  was  the  great- 
est pioneer  in  the  utilization  of  solar  heat.  He  began 
his  experiments  prior  to  1860  and  continued  them 
for  about  twenty  years  with  aid  from  the  French  gov- 
ernment. He  constructed  solar  cooking  appliances, 
and  later  large  machines  for  pumping  water  which  he 
installed  in  Algeria.  Mouchot  published  in  1869  a 
work  entitled  "La  Chaleur  Solaire  et  ses  Applica- 
tions Industrielles. "  A  second  edition  appeared  in 
1879.  He  gives  a  history  of  the  art,  describes  many 
applications  of  solar  heat,  and  summarizes  his  own 
work,  including  illustrated  descriptions  of  his  great 
solar  engines,  and  a  report  of  his  mission  to  Algeria  to 
install  for  the  Government  solar  pumping  plants  in 
the  desert  regions. 

Solar  heaters  after  the  general  form  of  Mouehot's, 
that  is  to  say,  with  a  conical  or  paraboloidal  reflector, 
and  glass-encased  tubular  boiler,  were  also  con- 
structed after  the  designs  of  M.  Pifre.  One  of  these 
was  exhibited  at  the  Tuileries  Garden  in  Paris  in 

366 


UTILIZING  SOLAR  ENERGY 

1882,  in  combination  with  a  steam  engine  and  print- 
ing press,  and  many  copies  of  a  paper  called  the 
"Soleil  Journal"  were  printed  by  solar  power. 

In  America  Captain  John  Ericsson,  the  inventor  of 
the  famous  " Monitor"  type  of  naval  vessels,  devised 
several  solar  engines,  1868  to  1886.  He  used  a  cylin- 
dric  mirror  of  parabolic  cross  section  to  concentrate 
the  rays  upon  a  tube.  A  two-and-a-half  horse-power 
engine  actuated  by  one  of  his  solar  heaters  was  ex- 
hibited in  New  York  at  the  American  Institute  Fairs 
for  several  years. 

ENEAS  SOLAR  ENGINES 

Fig.  68  shows  the  solar  machine  of  A.  G.  Eneas 
(U.  S.  Patents  No.  670,916  and  670,917  of  March  26, 


FIG.  68. — ENEAS'  SOLAR  ENGINE. 

1901).    One  of  his  solar  generators  was  in  use  for  a 
time  at  the  Cawston  Ostrich  Farm  near  Pasadena, 

367 


THE  SUN 


California,  and  others  in  Arizona,  for  pumping  water. 
The  mirror  is  composed  of  facets  of  silvered  glass  ar- 
ranged upon  the  inner  surface  of  a 
hollow  truncated  cone,  whose  sides 
make  an  angle  of  45°  to  the  axis. 
The  larger  diameter  of  the  cone  is 
stated  as  preferably  as  large  as 
thirty-two  feet,  and  in  several  in- 
stances was  actually  thirty-six  feet. 
Decided  advantage  is  claimed  in 
leaving  the  lower  end  of  the  mirror 
open,  as  it  greatly  diminishes  the 
wind  pressure,  and  the  part  of  the 
cone  omitted  is  not  very  useful  for 
gathering  heat.  The  mounting  shown 
in  the  first  patent  is  neither  equa- 
torial nor  alt-azimuth,  but  this  fea- 
ture was  improved  in  the  second 
by  substitution  of  the  equatorial 
form.  A  canvas  shield  was  provided 
to  protect  the  instrument  from  rain. 
An  interesting  feature  is  the  form  of 
construction  of  the  boiler  shown  in 
Fig.  69.  The  solar  rays  are  focussed 
upon  the  tube  F,  and  the  enlarged 
parts,/1  and/2,  are  respectively  above 
and  below  the  focal  region.  The  up- 
per enlargement  is  a  steam  and  water  drum,  the  lower 
a  settling  chamber  for  extracting  foreign  matter  from 
the  water.  Two  concentric  copper  tubes,  /  and  /8, 

368 


FIG.  69.— BOILER 
OF  ENEAS'  EN- 
GINE. 


UTILIZING  SOLAR  ENERGY 

connect  the  two  enlarged  chambers,  so  that  the  water 
falls  in  /  and  rises  in  /8,  the  latter  being  of  course 
the  hotter.  The  tube  /8  is  enclosed  by  one  or  more 
glass  tubes,  /n,  /12,  whose  purpose  is  to  retard  the 
escape  of  heat  from  /8  while  admitting  the  solar  rays. 

Mr.  Eneas  has  been  good  enough  to  furnish  me  the 
following  details  as  to  the  construction  of  his  ma- 
chines and  their  efficiency  in  actual  operation  : 

"Feb.  14,  1901.  Pasadena,  California,  llh  30m 
to  12h  30  m.  Cross-sectional  area  of  sunshine  inter- 
cepted 642  square  feet.  Air  temperature  61°  F. 
Steam  pressure  per  square  inch  145-151  pounds. 
Steam  condensed  123  pounds. 

"Oct.  3,  1903.  Mesa,  Arizona.  Cross  sectional 
area  of  sunshine  intercepted  700  square  feet.  Air 
temperature  74°  F.  Average  steam  pressure  per 
square  inch  141  pounds.  Steam  condensed  per  hour 
133  pounds.  Time  about  midday. 

"Oct.  9,  1904.  Wilcox,  Arizona.  Time  11  A.M.  to 
12  M.  -  Cross  sectional  area  of  sunshine  intercepted 
700  square  feet.  Steam  pressure  per  square  inch 
148-156  pounds.  Steam  condensed  144.5  pounds. 

"The  engines  used  were  of  the  fore  and  aft  com- 
pound condensing  marine  type,  complete  with  direct 
connected  air  and  feed  pump.  Size  224"  X  6"  X 

2  "  and  operated  at  460  to  520  revolutions  per  min- 


ute, with  about  ~-  cut  off  and  25"  to  26"  vacuum. 
16 

The  steam  used  in  the  engine  was  superheated  about 
40°  F.  in  the  later  machines. 

369 


THE  SUN 

"I  find  3.71  British  Thermal  Units  per  square  foot 
per  minute  given  as  the  greatest  amount  of  heat  ob- 
tainable during  the  trial  runs.  The  machines  re- 
sembled in  design  Patent  No.  670,917  with  equatorial 
mounting.  In  the  1904  model  the  greatest  and  least 
diameters  of  the  mirror  were  36  and  19  feet,  and  the 
angle  of  inclination  between  its  axis  and  its  sides  45°. 
The  mirrors  were  made  of  white  glass  similar  to  what 

Chance  Brothers  of  London  make,  and  were  about  — 

inch  thick,  18  inches  wide,  and  24  inches  long,  and 
were  sprung  to  the  curvature  of  the  frame.  White 
glass  was  used  to  reduce  the  loss  from  absorption. 
The  area  of  sunshine  intercepted  is  the  net  area  after 
deducting  for  shadows  caused  by  the  tension  rods 
and  frame  work.  In  the  later  machines  built,  the 
mirrors  were  set  so  as  to  concentrate  the  reflected 
rays  on  two  parts  of  the  boiler  instead  of  its  entire 
length  as  in  the  Pasadena  machine.  This  change 
gave  better  results"  (perhaps  because  of  the  better 
protection  of  the  remaining  parts  of  the  boiler  by 
non-conducting  wrapping  instead  of  glass  tubes). 
"The  total  cost  of  the  machine  complete  with  engine 
and  pump  was  $2,160. 

"An  average  day's  run  at  Wilcox  gave  results 
about  as  shown  in  table  on  following  page.  Date 
October  14,  1904. 


370 


UTILIZING  SOLAR  ENERGY 


TIME  HOURS 

7  A.M.    FOCUSSED 

Steam  Pressure 
in  Pounds 

Inches  on 
Water  Gauge 

8 

120 

14 

9 

125 

18 

10 

136 

21 

11 

140 

26^ 

12 

152 

30+ 

1  P.M. 

146 

30  + 

2 

141 

30  + 

3 

126 

28 

4 

83 

23 

5 

51 

10 

"  Vacuum  23.  Gallons  of  water  pumped,  146,780. 
Total  lift  plus  friction  39.4  feet. " 

(This  test  would  indicate  an  average  horse  power 
for  the  whole  day  of  about  2^.  From  data  to  be 
given  below  it  has  been  computed  that  this  means  the 
transformation  of  about  four  per  cent  of  the  solar 
radiation  intercepted  by  the  mirror  into  mechanical 
work.  From  coal  the  best  engines  transform  from 
twelve  to  fifteen  per  cent  of  the  heat  of  combustion 
into  mechanical  work,  but  probably  not  in  so  small  a 
plant  as  this.  The  result  of  course  depends  on  the 
efficiency  of  the  steam  engine  used,  as  well  as  on  that 
of  the  boiler.) 

Mr.  Eneas  continues: 

"  As  a  result  of  my  experience  with  about  nine  dif- 
ferent types  of  large  reflectors,  I  believe:  (1)  That 
with  similar  mirrors  perfected  in  details  about  3.90 
British  Thermal  Units  per  square  foot  per  minute 
would  be  the  greatest  amount  of  heat  obtainable  at 
noontime  in  Arizona  and  -other  cloudless  regions  of 

371 


THE  SUN 

similar  latitude.  (2)  That  better  progress  in  utiliz- 
ing solar  heat  commercially  for  power  can  be  made 
along  lines  described  in  the  Engineering  News  of  May 
13,  1909.  But  the  actual  obtaining  of  any  great 
amount  of  power  from  solar  rays  is  still  an  unsolved 
problem. " 

If  we  take  the  number  of  calories  per  square  centi- 
meter per  minute  available  as  1.4,  we  find  from  Mr. 
Eneas,  figure  of  3.71  British  Thermal  Units  per  square 
foot  per  minute  as  the  "  greatest  amount  of  heat  ob- 
tainable during  the  trial  runs"  that  about  seventy- 
two  per  cent  of  the  solar  radiation  was  turned  into 
heat  in  steam.  His  estimated  maximum  possible 
number  (3.90  B.  T.  U.)  corresponds  to  seventy-six 
per  cent.  This  is  really  a  very  satisfactory  result. 
The  maximum  steam  pressures  recorded  correspond 
to  a  temperature  of  about  185°  C. 

PROPERTIES  OF  GLASS 

The  use  of  one  or  more  glass  casings  as  an  adjunct 
to  the  boiler  of  the  Eneas  solar  engine  is  quite  analo- 
gous to  the  use  of  glass  by  de  Saussure,  Herschel,  and 
Mouchot,  and  also  to  its  common  use  by  gardeners 
over  their  hotbeds.  Glass  transmits  radiation  very 
freely  between  wave  lengths  0.37/z,  in  the  ultra-violet 
and  2.5/4  in  the  infra-red.  This  range,  as  indicated 
by  Fig.  26,  includes  nearly  all  the  solar  radiation. 
The  interposition  of  a  single  thin  glass  plate  in  a 
beam  of  sunlight  diminishes  the  intensity  about  fif- 
teen per  cent.  This  decrease  is  owing  principally  to 

372 


UTILIZING  SOLAR  ENERGY 

reflection.  The  rays  emitted  by  the  outside  of  the 
boiler,  if  we  estimate  its  temperature  at  500°  abso- 
lute Centigrade,  would  have  their  wave  length  of 
maximum  intensity  at  about  6/*  and  would  be  almost 
wholly  prevented  from  directly  escaping  as  radiation 
by  the  glass.  A  large  fraction  would  suffer  "  metallic 
reflection"  by  the  glass  back  to  the  boiler  tube,  and 
the  remainder,  being  absorbed  in  the  glass  itself, 
would  tend  to  raise  its  temperature  and  that  of  the 
air  space,  and  so  to  diminish  convection  from  the 
boiler  to  the  glass.  Furthermore,  the  glass  also  pre- 
vents wind  from  blowing  on  the  boiler,  and  cuts  off 
all  direct  convection  of  heat  to  the  outside  air,  which 
is  fully  as  valuable  a  function  as  the  restraint  of  out- 
ward radiation.  Thus,  the  employment  of  the  glass 
greatly  promotes  the  efficiency  of  the  device,  for  it 
raises  decidedly  the  temperature  of  the  boiler.  We 
shall  notice  below  the  connection  between  tempera- 
ture and  the  possible  thermodynamic  efficiency  of  the 
engine. 

We  have  already  given  the  interesting  story  told 
by  Sir  John  Herschel  of  the  dinner  he  cooked  under 
glass  by  solar  heat.  The  late  Secretary  S.  P.  Lang- 
ley  was  greatly  interested  by  this  story  and  had  more 
than  one  "hot  box"  constructed  on  similar  princi- 
ples. The  writer  designed  one  of  them.  It  con- 
sisted of  two  round  shallow  wooden  boxes,  the  inner 
one  50  centimeters  in  diameter,  the  outer  60  centi- 
meters, placed  concentrically  one  within  the  other 
and  each  covered  by  a  tightly  fitting  glass  plate.  The 

373 


THE  SUN 

boxes  were  further  protected  by  a  layer  of  feathers 
about  10  centimeters  thick  all  around  the  sides  and 
back  of  the  outer  box.  The  inner  one  had  a  black- 
ened metal  sheet  near  its  bottom,  and  suspended  a 
little  above  this  a  blackened  thermometer.  The 
whole  device  was  mounted  equatorially  and  kept 
toward  the  sun.  On  November  4,  1897,  at  Washing- 
ton, operating  with  three  glass  plates,  the  thermom- 
eter reached  118°  C.  while  the  outside  temperature 
was  16°  C. 

The  question  might  be  asked  whether  much  higher 
temperatures  are  not  practicable  to  attain  in  such  a 
manner  without  mirrors  or  lenses  to  concentrate  the 
heat.  Perhaps  with  better  construction  it  might  be 
possible  even  to  reach  200°  C.  with  such  contrivances. 
The  limiting  temperature  is  reached  when  the  solar 
heat  introduced  is  balanced  by  the  escape  of  heat  by 
conduction  through  the  glasses  and  through  the  in- 
sulating material  at  the  back.  The  effective  losses 
diminish  with  increasing  thickness  of  insulating  ma- 
terial, increasing  area  of  the  "hot  box,"  and  increas- 
ing numbers  of  glass  plates.  But,  unfortunately,  the 
increase  of  the  number  of  glass  plates  diminishes  the 
quantity  of  solar  radiation  reaching  the  inner  cham- 
ber, so  that,  as  found  by  de  Saussure,  two  or  three 
glasses  give  best  results.  The  writer  has  tested  with 
the  following  results  the  effect  of  introducing  in  a 
beam  of  sunlight  at  normal  and  also  45°  incidence 
successive  plates  of  the  common  glass  1.5  to  2.0  milli- 
meters thick  used  for  8X 10  photographic  plates,  and 

374 


UTILIZING  SOLAR  ENERGY 


of  plates  8  to  10  millimeters  thick  used  for  instrument 
covers: 

Percentage  transmission  of  glass  plates. 


Normal 

45° 

NUMBER  OF  PLATES  

1 

2 

3 

4 

1 

2 

3 

4 

49.0 

Thin  glasses  

86.5 

74.5 

63.5 

53.3 

85.0 

71.8 

60.0 

Thick  glasses  

79. 

34. 

50. 

39. 

SOLAR  HEATERS  AND  RESERVOIRS 

U.  S.  Patent  No.  230,323  of  July  20,  1880,  was  is- 
sued to  Messrs.  Molera  and  Cebrian,  who  proposed  to 
omit  the  costly  and  intricate  optical  devices  for  con- 
centrating the  solar  heat  as  used  by  Mouchot,  Erics- 
son, and  others,  and  even  the  mechanical  devices  for 
presenting  the  heater  broadside  toward  the  sun.  They 
proposed  a  horizontal  boiler  composed  either  of  a 
large  number  of  blackened  tubes  laid  side  by  side, 
or  a  pair  of  plates  enclosing  a  thin  stratum  of  liquid, 
and  communicating  in  either  case  with  a  suitable 
engine  designed  for  working  at  low  temperatures. 
These  inventors  make  no  mention  of  a  glass  cover  for 
their  boiler,  but  its  introduction  would  undoubtedly 
have  increased  the  efficiency  of  their  apparatus  very 
greatly. 

The  erection  upon  the  roof  of  a  building  of  a  series 
of  water  tanks  protected  by  a  non-conducting  mate- 
rial at  the  back,  and  by  a  glass  cover  above,  and  com- 
municating with  the  water  system  of  the  bath,  is 
26  375 


THE  SUN 

much  used  in  Southern  California,  and  doubtless 
elsewhere,  as  a  means  of  providing  warm  water.  Such 
devices  ordinarily  furnish  a  considerable  supply  of 
water  too  hot  to  bear  the  naked  hand  in,  and  save  the 
discomfort  of  fire  in  warm  weather. 

In  all  countries  the  sun  is  obscured  more  or  less  of 
the  time  by  clouds  and  during  the  night  hours,  so 
that  various  inventors  have  proposed  the  combina- 
tion of  a  device  for  gathering  solar  heat  and  a  large 
heat  reservoir,  which  usually  takes  the  form  of  a  tank 
of  water  having  non-conducting  walls,  and  is  situated 
above  the  level  of  the  heater,  to  which  it  communi- 
cates by  pipes.  U.  S.  Patent  No.  784,005  of  Feb.  28, 
1905,  to  E.  C.  Ketchum  recognizes  such  features  in 
combination  with  a  vaporizing  chamber  situated 
within  the  reservoir,  and  containing  some  vaporizable 
liquid  suitable  for  running  a  low  temperature  engine. 
In  the  event  of  a  very  prolonged  cloudy  spell  the  in- 
ventor proposes  also  a  furnace  for  heating  the  reser- 
voir independently  of  the  sun. 

Low  TEMPERATURE  SOLAR  -ENGINES 

Within  the  last  ten  years,  at  least  two  serious  at- 
tempts have  been  made  to  devise  commercially  eco- 
nomical means  of  employing  the  hot-box  principle  for 
power.  Both  series  of  experiments  are  described  in 
the  Engineering  News  for  May  13,  1909,  referred  to 
by  Mr.  Eneas.  The  inventors  are  Mr.  F.  Shuman,  of 
Philadelphia,  and  Messrs.  H.  E.  Willsie  and  J.  Boyle, 
Jr.,  of  Cranford,  N.  J.  The  Shuman  heat  absorber  is  a 

376 


UTILIZING   SOLAR   ENERGY 

level  hard  rolled  plot  of  ground  rendered  waterproof 
by  covering  it  with  asphaltum  and  enclosed  by  plank 
partitions  rising  a  few  inches  above  the  bottom.  In 
this  tank  water  is  filled  to  a  level  of  about  three  inches 
and  over  it  a  thin  layer  of  paraffin,  which  of  course 
melts  in  the  sun,  and  hinders  evaporation  and  radia- 
tion from  the  water  surface,  while  transmitting  the 
solar  rays  to  the  water  and  asphaltum.  The  whole 
tank  is  tightly  covered  with  a  single  layer  of  glass  set 
in  oiled  cotton  packing.  Wind  screens  are  erected 
to  protect  the  tank  from  convection  losses.  The  cost 
of  such  construction  is  said  not  to  exceed  twenty-five 
cents  per  square  foot,  and  it  is  expected  to  produce  a 
horse  power  for  each  160  square  feet  (it  is  not  stated 
if  this  is  the  average  of  all  conditions  or  only  the  re- 
sult in  the  most  favorable  hours,  but  almost  certainly 
it  is  the  latter) .  The  water  flows  from  the  heater  to  a 
steam  turbine  operated  in  connection  with  a  vacuum 
pump.  Assuming  an  initial  temperature  of  202°  F.,  the 
vacuum  causes  the  explosion  of  perhaps  ten  per  cent 
of  water  into  steam  and  the  reduction  of  the  tem- 
perature of  the  mixed  steam  and  water  to  about 
102°  F. 

As  the  maximum  possible  thermodynamic  effi- 
ciency under  such  conditions  is  fifteen  per  cent,  it  is 
unlikely  that  as  much  as  five  per  cent  of  the  sun's 
heat  can  be  converted  into  mechanical  work.  A 
large  storage  reservoir,  built  below  ground  and  well 
insulated,  is  connected  with  the  apparatus  in  such  a 
manner  that  the  excess  of  hot  water  during  the  hot- 

377 


THE  SUN 

test  part  of  the  day  goes  in  at  the  top  of  the  reservoir, 
while  water  from  the  bottom  of  the  reservoir  is  with- 
drawn to  supply  that  withdrawn  from  the  heating 
tank.  During  the  morning  and  evening  hours,  or 
under  cloudy  conditions,  the  motor  can  be  run  from 
the  reservoir.  The  plant  is  still  in  the  experimental 
stage,  but  appears  to  be  well  planned,  and  promises 
considerable  success. 

The  apparatus  of  Messrs.  Willsie  and  Boyle  has 
been  more  thoroughly  tried,  so  that  Mr.  Willsie  gives 
actual  figures  as  to  cost  and  efficiency.  They  prefer 
to  build  an  entirely  wooden  basin  coated  with  asphalt, 
for  they  find  the  sand  even  of  the  desert  to  contain 
moisture  which  injures  its  quality  for  a  non-conductor 
of  heat.  In  order  to  promote  a  more  rapid  circula- 
tion of  the  water,  and  its  consequent  higher  efficiency 
to  absorb  heat,  they  incline  the  basin.  In  their  latest 
construction  the  water  runs  from  a  first  basin  with 
one  glass  cover  to  a  second  with  two,  and  from  this  it 
drips  over  a  row  of  pipes  containing  sulphur  dioxide 
gas.  They  employ  a  low  pressure  sulphur  dioxide 
engine  of  the  type  developed  *n  Germany  by  Pro- 
fessor Josse.  In  their  experiments  they  run  between 
temperatures  approximating  200°  F.  and  100°  F.,  but 
at  midday  their  heater  sometimes  reaches  nearly  260° 
F.  They  also  combine  their  apparatus  with  a  large 
reservoir  for  use  at  night  or  in  cloudy  weather.  Four 
installations  have  been  erected  by  Willsie  and  Boyle, 
the  first  at  the  St.  Louis  Exposition,  the  others  at 
Needles,  Arizona,  a  place  that  all  travelers  who  have 

378 


UTILIZING   SOLAR   ENERGY 

been  there  will  agree  is  well  qualified  for  experiments 
with  solar  heat!  Mr.  Willsie  estimates  the  cost  of 
installing  a  sun-power  plant  at  $164  per  horse  power, 
and  the  cost  of  operating  400-horse-power  steam- 
electric  and  solar-electric  plants  in  desert  regions  at 
2.08  and  0.61  cents  per  electric  horse-power  hour, 
respectively. 

SOLAR  COOKING  APPLIANCES 

Experiments  in  solar  cooking  which  attracted  con- 
siderable public  attention  were  made  in  1878  by  W. 
Adams  of  Bombay,  India.  Fig.  70  shows  the  very 
simple  apparatus  employed  by  him  for  cooking  pur- 
poses. The  eight-sided  conical  concentrator  was 
made  of  wood  lined  with  silvered  glass.  It  was 
hinged  upon  a  board  and  adjusted  by  a  wedge  and  by 
rotating  the  board  so  as  to  face  the  sun.  The  posi- 
tion of  the  apparatus  required  to  be  changed  about 
once  each  half  hour.  The  cooking  vessel  of  copper 
was  enclosed  in  a  glass  case  and  fixed  to  the  back  of 
the  concentrator.  Mr.  Adams  wrote  to  the  Scientific 
American,1  that  the  rations  of  seven  soldiers,  consist- 
ing of  meat  and  vegetables,  were  thoroughly  cooked 
by  it  in  a  couple  of  hours,  in  January,  the  coldest 
month  of  the  year  in  Bombay;  and  that  the  men  de- 
clared the  food  to  be  cooked  much  better  than  in  the 
ordinary  manner.  It  was  also  tried  with  success  by 
several  people  in  Bombay  and  in  the  Deccan.  The 


1  June  5,  1878. 
379 


THE  SUN 


380 


UTILIZING  SOLAR  ENERGY 

dish  is  stewed  or  baked,  according  as  the  steam  is  re- 
tained or  allowed  to  escape.  Adams'  reflector  was 
two  feet  four  inches  in  diameter. 

SOLAR  METALLURGY 

Besides  devices  for  producing  power,  for  cooking, 
and  for  warming  water  for  domestic  purposes,  by 
solar  heat,  we  may  note  its  proposed  application  for 
metallurgy.  U.  S.  Patent  No.  277,884  of  May  22, 
1883,  was  granted  to  John  Clark  of  England  for  a 
"  Method  of  Reducing  Metals  from  their  Ores. "  The 
inventor  proposes  to  use  a  concave  mirror  built  of 
segments  of  silvered  glass  or  burnished  metal, 
mounted  in  a  manner  convenient  to  face  the  sun,  and 
adapted  to  focus  the  solar  rays  upon  a  stick  of  ore, 
for  instance  of  the  oxide  or  chloride  of  aluminum  or 
magnesium,  formed  into  a  convenient  shape  by  com- 
pression from  the  powdered  substance.  He  proposes 
either  to  mix  solid  reducing  agents  with  the  ore  or 
else,  when  the  ore  is  heated  to  a  suitable  temperature, 
to  convey  a  gaseous  reducing  agent,  as  hydrogen  or 
carbon  monoxide,  to  the  incandescent  material.  The 
excess  of  the  reducing  agent  is  supposed  to  prevent 
reoxidation  of  the  reduced  metal,  but  this  may  be 
further  guarded  against  by  enclosing  the  whole  ap- 
paratus in  a  glass  roofed  chamber  filled  with  a  neu- 
tral or  reducing  gas.  The  advantage  claimed  for  the 
proposed  use  of  solar  rather  than  other  sources  of 
heat  is  the  fact  that  a  very  high  temperature  can  thus 
be  readily  obtained. 

381 


THE  SUN 

RESUME 

In  the  preceding  pages  we  have  noted  various 
devices  which,  singly  or  in  combination,  have  been 
employed  by  numerous  inventors  for  the  utilization 
of  solar  heat.  They  comprise  first  a  large  surface 
for  receiving  the  sun's  rays.  This  may  be  fixed  in  a 
horizontal  or  other  preferred  position,  or  progress- 
ively inclined  by  suitable  mechanism  to  suit  the  posi- 
tion of  the  sun.  In  the  former  case  the  surface  is 
blackened  to  promote  absorption,  and  the  heat  thus 
derived  is  communicated  to  some  liquid  for  domestic 
use  or  for  the  running  of  a  low  temperature  heat  en- 
gine. More  commonly  mirrors  (or  sometimes  lenses 
or  prisms)  are  provided  for  concentrating  the  rays  to 
an  approximate  focus.  Usually  the  mirror  is  com- 
posed of  a  large  number  of  facets  of  plane  silvered 
glass  or  burnished  metal  arranged  upon  a  frame  of 
suitable  general  curvature.  The  form  of  the  reflect- 
ing combination  may  be  a  paraboloid,  or  cone  of  rev- 
olution, or  an  arc  of  a  cylinder  of  parabolic  cross  sec- 
tion. At  the  approximate  center  of  concentration 
of  the  rays  is  located  a  heater  for  the  ore  to  be  reduced 
or  the  liquid  to  be  vaporized.  Advantage  is  gained  in 
this  case,  and  also  in  the  fixed  forms  of  solar  heater, 
by  encasing  the  heated  part  with  glass  in  the  direc- 
tion from  which  come  the  solar  rays,  and  protecting 
it  by  non-conductors  of  heat  in  other  directions.  The 
means  of  presenting  apparatus  to  the  sun  usually  em- 
ployed by  astronomers,  such  as  the  English  type  of 

382 


UTILIZING  SOLAR  ENERGY 

open  fork  equatorial  mounting,  which  would  seem  to 
be  excellently  adapted  for  the  purpose,  do  not  appear 
to  have  appealed  to  the  solar  engine  inventors,  as  a 
rule.  They  have  generally  devised  more  complex 
mechanical  movements  for  their  purposes,  including 
circular  tracks,  slotted  hinged  uprights,  intermediate 
types  between  the  alt-azimuth  and  equatorial  forms 
of  mounting,  etc.  Excepting  the  solar  heaters  for 
bath  purposes  commonly  installed  in  the  roofs  of 
houses,  it  does  not  appear  that  appliances  for  utilizing 
solar  heat  are  yet  introduced  with  economical  success 
in  practice,  for  although  much  work  has  been  done  in 
this  line  for  centuries,  we  hardly  ever  see  any  of  the 
machines. 

We  shall  conclude  this  chapter  by  a  consideration 
of  some  of  the  data  to  be  used  in  the  design  of  solar 
heat  apparatus. 

QUANTITY  OF  SOLAR  ENERGY  AVAILABLE 

We  may  first  inquire  how  much  solar  radiation  is 
available.  The  following  data  are  computed  from 
the  Smithsonian  pyrheliometric  observations  at 
Washington  and  Mount  Wilson.  Sun  rays  may  be 
received  on  a  surface  at  right  angles  to  the  beam 
(" normal  incidence")?  m  which  case  the  surface  must 
be  moved  by  suitable  mechanism  to  follow  the  ap- 
parent motion  of  the  sun  in  the  heavens.  On  the 
other  hand,  the  rays  may  be  received  on  a  fixed  hori- 
zontal surface,  in  which  case  their  intensity  will 
diminish  as  the  cosine  of  the  sun's  zenith  distance. 

383 


THE  SUN 


\ 


\ 


In  either  case  there  is  the  decrease  of  the  intensity  of 
the  rays  depending  on  the  length  of  path  in  the  at- 
mosphere. Fig. 
71  gives  the 
mean  intensity 
of  direct  sun- 
shine in  calo- 
ries per  square 
centimeter  per 
minute  for 
Mount  Wilson 
and  Washing- 
ton. Horizon- 
tal distances 
give  "  air  mass- 
es, "  or,  in  other 
words,  secants 
of  the  zenith 
distances  of  the 
sun1.  Vertical 
distances  are 
calories.  One 
pair  of  curves, 
III  and  IV,  is 
for  the  receiv- 
I  and  II,  for 


\ 


5 

(Mount 


"I  SEC.Z     234 

FIG.  71.  —  INTENSITY  OF  SUN  RAYS. 
Wilson  and  Washington.) 
I,  II.  Normal  incidence.     Ill,  IV.  On 
horizontal  surface. 

ing   surface    horizontal,   the  others, 


1  The  secant  of  the  zenith  distance  ceases  to  represent  closely  the 
"air  mass"  for  zenith  distances  above  78^°  where  sec.  Z  =  5.  From 
some  measurements  made  at  very  low  sun  the  data  given  below  are 
extended  to  sun  rising  and  setting. 

384 


UTILIZING   SOLAR   ENERGY 

" normal  incidence."    The  curves  I  and  III  are  for 
Mount  Wilson. 

In  Fig.  72  (upper  half)  is  shown  for  sea,  level  and 
6000  feet  elevation,  both  for  horizontal  and  normal 
incidence,  the  march  of  the  sun's  direct  radiation 


\\ 


\\ 


u 


\\ 


\ 


\\ 


-?tfr 


400 


& 


20 


FIG.  72. — INTENSITY  OF  SOLAR  RADIATION. 

Sea-level   and    6,000-feet   elevation.     Normal   incidence   and   on   hori- 
zontal surface. 

from  noon  to  sunset  on  December  22,  February  17 
(and  October  25),  March  21  (and  September  23), 
April  22  (and  August  22),  June  22,  at  which  times 
the  sun's  declination  is— 23^°— 12°,  and  0°,  +  12° 
-f  23J/2°,  respectively.  The  data  are  computed  for 
latitude  38°  N.  Horizontal  distances  give  the  hours, 

385 


THE  SUN 


and  vertical  distances  the  calories  per  square  centi- 
meter per  minute.  Similar  computations  have  been 
made  for  latitudes  20°  N.,  30°  N.,  and  45°  N.,  but 
are  not  shown  in  Fig.  72.  From  these  results  come 
the  data  represented  in  the  lower  half  of  Fig.  72.  The 
curves  show  the  number  of  calories  of  solar  heating 
per  square  centimeter  per  day  falling  in  cloudless 
weather  on  surfaces  at  horizontal  and  normal  inci- 
dence, at  sea-level  and  6000  feet  altitude  respectively 
for  the  given  latitudes.  In  each  group  the  two 
upper  curves  are  for  normal  incidence;  the  highest 
for  6000  feet  elevation.  In  the  following  table  is  a 
summary  of  the  whole  matter  expressed  in  calories 
per  square  centimeter  per  year,  and  also  in  square 
feet  required  on  the  average  per  horse-power  assum- 
ing complete  absorption  and  transformation,  and  the 
sun  to  shine  261,000  minutes  per  year. 


Latitude 

NORMAL  INCIDENCE 

HORIZONTAL  SURFACE 

Sea-level 

6,000  feet 

Sea-level 

6,000  feet 

20° 

30° 
38° 
45° 

292,000 
287,000 
271,000 
270,000 

362,000 
355,000 
342,000 
340,000 

185,000 
170,000 
152,000 
137,000 

226,000 
•  203,000 
185,000 
169,000 

Calories  per  sq. 
cm.  per  year 

20° 
30° 

38° 
45° 

10.5 
10.7 
11.3 
11.4 

8.5 
8.8 
9.0 
9.1 

16.6 
18.1 
20.2 
22.4 

13.6 
15.1 
16.6 

18.2 

Average  sq.  feet 
per  horse- 
power 

It  is  not  difficult  to  absorb  ninety-five  per  cent  of 
the  solar  radiation  falling  upon  a  surface.     Lamp- 

386 


UTILIZING  SOLAR  ENERGY 

black  is  employed  as  an  absorber  if  the  temperature 
is  low,  and  platinum  black  electrolytically  deposited 
if  the  temperature  is  so  high  as  to  burn  off  lampblack. 
There  are  many  regions  of  the  earth  where  the  days 
are  seventy-five  to  ninety  per  cent  cloudless,  or  even 
more.  Hence  we  may  conclude  that  there  are  many 
regions  where  for  the  average  daylight  hours  it  is 
practicable  to  absorb  on  a  surface  of  from  one  to  two 
square  yards  solar  heat  mechanically  equivalent  to  a 
horse-power.  But  in  the  production  of  mechanical 
power  from  solar  heat  only  a  small  percentage  is  ac- 
tually utilized. 

THERMODYNAMIC  EFFICIENCY 

It  is  shown  in  works  on  Thermodynamics  that  a 
perfect  engine  taking  in  heat  at  the  absolute  temper- 
ature TX,  and  rejecting  it  at  T2,  can  transform  only 

rp      rp 

the  fraction  --1          2  of  the  heat  into  mechanical 
±1 

work.  For  illustration,  suppose  the  engine  taking  in 
heat  at  the  boiling  point  of  water,  373°  C.  absolute, 
and  rejecting  it  at  the  freezing  point,  273°,  the  maxi- 

100 

mum  efficiency  possible  will  then  be  ^rr  =  26.8  per 

o7o 

cent.  This  thermodynamic  law  gives  the  efficiency 
of  a  perfect  engine,  and  it  does  not  matter  what  its 
nature,  if  its  actuating  energy  is  heat.  A  thermo- 
electrical  engine  or  a  steam  engine  are  both  heat  en- 
gines, and  their  efficiency  cannot  exceed  that  calcu- 
lated by  the  above  rule.  In  fact,  however,  no  heat 

387 


THE  SUN 


engine  is  perfect,  and  the  best  triple  expansion  con- 
densing steam  engines,  with  the  best  constructed 
boilers,  hardly  ever  convert  as  much  as  fifteen  per 
cent  of  the  heat  of  combustion  of  the  coal  they  con- 
sume into  work.  If  a  heat  engine  works  from  a  very 

rp      _     rp 

high  temperature  to  a  low  one,  the  fraction     l 

M 

may  approach  nearly  to  unity.     For  instance,  sup- 


pose T!  =  1000°  and  T2  =  300°  then 


*i 


=  70 


per  cent.  This  accounts  in  part  for  the  high  effi- 
ciency of  internal  explosion  engines,  which  develop 
high  temperatures  in  their  cylinders,  and  often  con- 
vert twenty-five  per  cent  of  the  heat  of  combustion  of 
their  fuel  into  work.  On  the  other  hand,  the  losses  of 
heat  by  conduction,  convection,  and  radiation  in- 
crease rapidly  with  rising  temperatures,  so  that  if 
engines  are  used  at  very  high  temperatures  the  ther- 
modynamic  gain  may  be  counterbalanced  by  a  prac- 
tical loss. 

REFLECTING  POWER  OF  MIRROR  SURFACES 

TABLE  XXX.  —  Percentage  reflecting  power  of  various  surfaces 


WAVE  LENGTH 

0.35/x 

0.4(V 

0.45M 

0.50/t 

0.60/m 

0.70/ut 

0.80ft 

LOOM 

1.50ft 

95 
65 

98 
79 
75 

Glass1,  silvered  on  back. 
Sample  A  
Glass,  silvered  on  back, 
Sample  B      

67 
68 

83 
53 
55 

82 

80 
73 

90 
59 
60 

90 

86 
71 

91 
61 
63 

93 

84 
70 

93 
65 
64 

94 

76 
73 

95 
69 

67 

94 
65 

96 
70 

68 

95 
56 

97 
72 
70 

Glass,  mercury  back  .... 
Silvered  on  glass  (Chem. 
Dept.)           

74 
48 
51 

Nickel  (Electrolyt.  Dep.) 
Speculum  metal  

1  The  reflecting  power  of  mirrors  coated  on  the  back  differs  greatly 
with  the  character  of  the  glass  used.     Sample  A  is  ordinary  optical 

388 


UTILIZING  SOLAR  ENERGY 

Taking  all  things  into  consideration,  glass  plates 
silvered  on  the  back  are  probably  the  best  materials 
for  constructing  the  mirrors  of  solar  heaters. 

One  may  well  question  whether  the  solar  engine  of 
the  future  will  have  mirrors  and  driving  mechanism. 
The  "hot  box"  of  de  Saussure  and  Sir  John  Herschel 
as  applied  by  Willsie  and  Boyle  and  Shuman  is  so 
cheap  that  the  low  efficiency  inseparable  from  its  low 
working  temperature  seems  not  to  bar  its  use  com- 
mercially. Some  gain  in  efficiency  may  be  made  by 
installing  the  fixed  heating  surface  parallel  to  the 
earth's  axis  instead  of  horizontal,  but  perhaps  the 
increased  cost  may  offset  this  gain.  The  efficiency 
of  the  apparatus  depends  on  the  excellence  of  the 
glass  protection  in  front.  If^one  could,  in  addition, 
make  a  vacuum  under  the  glass  economically,  the 
efficiency  jvould  be  much  higher.  This  device  de- 
serves much  attention. 

It  seems  highly  probable  that  solar  cooking  uten- 
sils, combined  with  water  heaters  and  heat  reservoirs, 
and  embodying  the  "fireless  cooker"  principle,  will 
come  into  extensive  use.  For  it  is  not  hard  to  see 
that  very  inexpensive  apparatus  may  be  designed  for 
combining  these  utilities,  and  that  housekeepers  will 
welcome  a  relief  from  the  hot  kitchen  conditions  of 
summer. 

flint  glass,  12  millimeters  thick,  silvered  on  the  back  by  chemical 
deposition.  Sample  B  is  ordinary  commercial  plate  glass  of  a 
greenish  tinge,  about  8  millimeters  thick,  similarly  silvered.  The 
glass  of  sample  B  perhaps  has  an  absorption  band  in  the  upper  infra- 
red spectrum. 

389 


THE  SUN 

Good  designing,  avoiding  costly  and  complicated 
construction  and  devices  likely  to  require  frequent 
attention,  combined  with  a  fuller  knowledge  of  the 
properties  of  materials  available,  and  cleverness  in 
adapting  means  to  promote  efficient  results, — these  if 
supported  by  a  moderate  outlay  of  money  for  experi- 
mental work  may  perhaps  soon  make  the  utilization 
of  solar  energy  very  extensive. 


CHAPTER  X 

THE   SUN   AMONG   THE   STARS 

Stellar  Distances. — Magnitudes. — Sun's  Magnitude  and  Light 
Emission. — Solar  Motion. — Star  Groups. — Double  Stars. —  Stel- 
lar Masses  and  Densities. — Mira  Ceti  and  the  Sun. — Stellar 
Spectra.  —  The  Classification  of  Stellar  Spectra.  —  Radiation 
Distribution. — Evolution  of  the  Solar  System. — Stellar  Evolu- 
tion. 

AT  first  glance  the  stars  appear  to  be  about  as  much 
like  the  sun  as  the  fireflies  of  a  summer  night.  It  is 
only  prolonged  investigation  which  has  proved  that 
the  sun  is  merely  a  star,  and  by  no  means  the  largest 
of  them;  and  that  if  the  sun  should  be  removed  a 
great  distance  it  would  appear  like  one  of  the  stars. 
The  Copernican  view  that  the  sun  is  the  center 
about  which  the  earth  and  planets  revolve  seemed 
satisfactory  enough  as  far  as  concerned  the  solar 
system,  but  was  for  centuries  hard  of  belief  as 
regards  the  stars.  For  it  required  the  assumption 
that  these  were  all  so  distant  that  the  enormous  dis- 
placement of  the  earth  in  space  between  summer  and 
winter  produced  no  measureable  changes  in  their  ap- 
parent relative  positions.  If  the  reader  will  walk  a 
hundred  paces  in  any  direction  within  a  forest,  he  will 
instantly  see  that  the  trees  change  their  relative  direc- 
tions from  him,  and  if  he  rides  in  the  cars  he  perceives 

27  391 


THE  SUN 

that  the  foreground  of  the  landscape  appears  to  re- 
volve. Objects  at  greater  and  greater  distances  from 
him  are  seen  to  be  less  and  less  apparently  affected 
among  themselves  by  his  motion.  Accordingly,  if  the 
astronomer  of  a  century  ago  agreed  with  Copernicus 
he  had  to  believe  that  since  the  stars  do  not  sensibly 
change  in  their  relative  positions  during  the  year, 
they  are  so  distant  from  the  earth  that  a  displacement 
of  between  one  and  two  hundred  million  miles  in  the 
position  of  the  earth  as  viewed  from  the  nearest  star 
subtended  an  angle  too  small  for  him  to  measure.  In 
other  words,  as  he  could  observe  changes  as  small  as  a 
couple  of  seconds  of  arc,  he  had  to  believe  that  the 
stars  were  at  any  rate  all  more  than  100,000,000  X 
100,000  miles  away.  In  our  day  we  know  that  this 
is  so,  for  the  distances  of  some  of  them  have  actually 
been  measured,  but  a  century  ago  the  astronomers 
took  it  on  faith,  merely  because  they  accepted  the 
Copernican  system. 

The  first  successful  measurements  of  stellar  pa  al- 
laxes  (a  star's  annual  parallax  is  the  angle  the  radius 
of  the  earth's  orbit  subtends  viewed  from  the  star) 
were  made  by  Struve,  at  Dorpat,  on  Vega,  1835  to 
1838,  and  by  Bessel  on  the  star  61  Cygni,  1837  to 
1840.  The  latter  faint  star  was  selected  on  account 
of  its  large  proper  motion.  Bessel's  result  was  0."35, 
and  Struve's  about  one-quarter  second.  The  former 
is  nearly  correct,  the  latter  about  twice  too  large.  It 
was  a  great  feat  to  measure  such  small  angles  as  these. 
In  modern  practice  the  efforts  to  measure  parallaxes 

392 


THE  SUN  AMONG  THE  STARS 

absolutely  has  practically  been  discontinued,  and  in- 
stead relative  parallaxes  are  determined.  That  is, 
instead  of  measuring,  for  instance,  the  apparent  ab- 
solute change  of  polar  distance  of  a  certain  star  due  to 
the  earth's  revolution  around  the  sun,  astronomers 
now  for  the  most  part  determine  how  much  a  given 
star  appears  to  shift  among  very  faint  neighboring 
stars  owing  to  the  same  cause.  For  it  is  now  as- 
sumed that  the  very  faint  stars  on  the  average  are  so 
very  distant  that  they  have  no  sensible  parallaxes, 
or  at  most  a  very  minute  and  approximately  known 
average  parallax,  which  can  be  applied  as  a  correc- 
tion. Stars  are  generally  selected  for  individual  par- 
allax measurements  because  they  have  relatively 
large  " proper  motions,"  or  progressive  apparent 
displacement  among  the  stars.  This  is  usually 
a  safe  criterion  of  comparative  nearness,  as  ap- 
pears from  our  illustration  of  the  railway  train 
above.  In  a  survey  of  ninety-two  stars  pub- 
lished a  few  years  ago  by  Chase  of  Yale,  there 
were  found  the  following  numbers  of  stars  between 
given  limits  of  parallax: 


Number  of  Stars 

Limits  of  Parallax 

2 

0".25to    0".20 

6 

0".20to    0".15 

11 

0".15to    O'MO 

24 

O'MOto    0".05 

34 

0".05to    0".00 

8 

0".  00  to  -0".  05 

5 

-0".  05  to  -O'MO 

2 

-0".  10  to  -0".  15 

THE   SUN 

The  negative  parallaxes  are  of  course  illusory,  and 
hence  we  may  suppose  that  some  positive  ones  are 
also  erroneous,  so  that  of  this  lot  no  more  than  two- 
thirds  have  measurable  parallaxes,  and  of  the  whole 
lot  three-fourths  are  more  distant  than  100,000,000  X 
2,000,000  miles.  It  is  customary  to  express  such 
enormous  distances  in  terms  of  "light  years. "  Light 
travels  in  one  year  about  6,000,000,000,000  miles. 
Accordingly,  the  number  given  above  is  about  thirty 
light  years,  which  represents  approximately  the  dis- 
tance of  a  star  whose  parallax  is  one- tenth  second. 
The  vast  majority  of  stars  are  many  times  as  distant 
as  this,  and  the  nearest  yet  found  is  a  Centauri,  whose 
distance  is  about  four  light  years. 

STELLAR  MAGNITUDES.     THE  SUN'S  MAGNITUDE 

The  relative  brightness  of  the  stars  is  expressed  in 
"  Magnitudes, "  a  star  of  the  first  magnitude  giving 
about  2.5  times  the  light  of  one  of  the  second.  On 
this  scale  Polaris  is  nearly  of  the  second,  Aldebaran 
nearly  of  the  first,  Vega  nearly  of  zero,  Sirius  —  1.4, 
and  the  sun  —  26.5.  A  change  of  five  magnitudes 
makes  a  change  of  one  hundredfold  in  the  light,  so 
that  the  sun  gives  the  earth  over  90,000,000,000  times 
the  light  of  Aldebaran.  If  removed  to  the  distance 
of  Aldebaran,  whose  annual  parallax  is  O."ll,  the  sun 
would  become  a  star  of  the  fifth  magnitude,  and  ap- 
pear only  about  as  bright  as  the  fainter  stars  among 
the  six  easily  seen  in  the  Pleiades.  Accordingly,  Al- 
debaran emits  about  forty-five  times  as  much  light 

394 


THE  SUN   AMONG   THE  STARS 

as  the  sun.  There  are  some  stars,  among  them  Rigel, 
Canopus,  and  Deneb,  which  are  sensibly  of  zero  par- 
allax and  yet  of  first  magnitude,  or  brighter,  so  that 
they  must  emit  many  thousands,  perhaps  hundreds 
of  thousands  of  times  as  much  light  as  the  sun.  On 
the  other  hand,  there  are  many  stars  whose  light 
emission  is  very  much  less  than  the  sun's,  among 
them  the  rapidly  moving  star  whose  parallax  was 
measured  by  Bessel,  sixty-one  Cygni.  Its  light  emis- 
sion is  one-tenth  that  of  the  sun. 


SOLAR  MOTION  AMOifa  THE  STARS 

As  in  a  forest  walk  the  trees  in  front  seem  to  sepa- 
rate as  we  approach,  and  those  behind  to  crowd  to- 
gether as  we  recede,  so  the  stars  exhibit  a  tendency  to 
move  from  the  approximate  direction  of  the  constel- 
lation Hercules  towards  the  constellation  Argo  in  the 
Southern  Hemisphere.  In  consequence  of  the  great 
distance  of  the  stars  these  displacements,  called  proper 
motions,  are  very  slow,  not  often  exceeding  100"  per 
century,  and  usually  very  much  less.  Nevertheless, 
the  observations  of  star  places  are  so  exact  that  the 
foci  of  the  motions  have  been  determined  with  an  un- 
certainty of  only  a  few  degrees.  That  in  the  North- 
ern Hemisphere  lies  in  R:ght  Ascension  270°,  Declin- 
ation +  30°,  in  the  constellation  Hercules  about  10° 
southwest  of  the  bright  star  Vega.  The  cause  of  the 
phenomenon  is  the  motion  of  the  solar  system,  rela- 
tively to  the  stars  in  general,  toward  the  position  just 
defined,  called  the  solar  apex.  The  rate  of  motion  is 

395 


THE  SUN 

determined  from  the  apparent  proper  motions  of  the 
stars  of  known  parallaxes,  or  from  the  general  spectro- 
scopic  survey  of  the  stellar  motions  in  the  line  of  sight. 

Professor  Campbell  has  been  good  enough  to  give 
me  the  following  summary  of  various  determinations : 

"Sir  William  Herschel  in  1783  deduced  from  the 
proper  motions  of  thirteen  stars  (all  then  available) 
that  the  solar  system  was  travelling  approximately 
toward  the  star  Lambda  Herculis  in  Right  Ascension 
262°,  Declination  +  26°  '. 

"  Many  determinations  of  the  goal  of  the  sun's  way 
were  made  in  the  latter  half  of  the  nineteenth  century, 
as  the  proper  motions  of  stars  became  known  in 
greater  numbers.  Of  those  based  upon  the  most  ex- 
tensive lists  of  proper  motions  we  mention  the  follow- 
ing: 

"  Newcomb's  coordinates  for  the  apex  of  the  sun's 
way,  deduced  from  about  3,100  Bradley  stars,  are 
Right  Ascension  275°,  Declination  +  31°  2. 

"  From  2,640  Bradley  stars,  Kapteyn  deduced  the 
position  of  the  apex  Right  Ascension  274°,  Declina- 
ation  +  29°.53. 

"  From  the  proper  motions  of  5,413  stars  Boss  has 
computed  the  apex  to  be  at  Right  Ascension  270°. 5, 
Declination  +  34°. 3;  and  his  estimate  of  the  velocity 
of  the  solar  motion  is  24  km.  per  second.4 

1  Philosophical  Transactions,  vol.  xv,  page  405,  1783. 

2  The  Stars,  page  91,  1901. 

3  Astronomische  Nachrichten,  vol.  olvi,  page  17,  1901. 

4  Astronomical  Journal,  No.  614,  1910. 

396 


THE   SUN   AMONG   THE   STARS 

"  Several  solutions  for  the  elements  of  the  solar 
motion  have  been  based  upon  the  observed  radial 
velocities  of  stars. 

"  In  1900  Campbell,  from  280  stellar  radial  veloci- 
ties in  the  northern  three-fifths  of  the  sky,  obtained 
for  the  position  of  the  apex  Right  Ascension  277°. 5, 
Declination  +  20°;  and  for  the  speed  19.9  km.  per 
second. 

"  In  1909  Hough  and  Halm  based  a  solution  upon 
about  500  stellar  radial  velocities.  They  obtained 
for  the  position  of  the  apex  Right  Ascension  27 1°, 
Declination  +  25°. 6;  and  for  the  velocity  20.85  km. 
per  second. 

"  In  1910  Campbell  deduced  the  elements  of  the 
solar  motion  from  the  observed  radial  velocities  of 
1034  stars  and  thirteen  nebulae.  His  position  of  the 
apex  was  deduced  as  Right  Ascension  272°,  Decli- 
nation +  27°. 5;  and  the  velocity  as  17.8  km.  per 
second. 

"  It  should  be  held  in  mind  that  the  motion  of  the 
solar  system  is  a  purely  relative  term,  and  in  every 
case  refers  to  the  particular  group  of  stars  used  as  a 
basis  for  the  solution.  The  computer's  aim  should 
always  be  to  have  his  observational  material  as  ho- 
mogeneous and  as  representative  of  the  entire  sidereal 
system  as  possible. 

"  It  appears  that  an  uncertainty  of  several  degrees 
exists  as  to  the  direction  of  the  solar  motion  with 
reference  to  the  entire  sidereal  system,  and  perhaps  of 
several  kilometers  as  to  the  speed  of  this  motion. 

397 


THE  SUN 

Perhaps  the  following  values  are  as  probable  as  any 
that  we  can  at  present  assign : 

"Apex  at  Right  Ascension  270°,  Decimation 
+  30°.  Velocity  18  km.  per  second. 

"It  should  be  said,  however,  that  some  astrono- 
mers would  consider  the  following  values  as  more 
probable : 

"Right  Ascension  272°,  Declination  +33°; 

"  Velocity  20  km.  per  second. 

"  Kapteyn  and  Frost  have  obtained  indications  that 
the  speed  of  the  solar  system,  with  reference  to  stars 
of  spectral  type  B,  is  considerably  greater  than  with 
reference  to  the  system  as  a  whole,  but  the  number  of 
B-type  stars  employed  in  the  discussion  is  perhaps 
too  small  to  yield  results  entirely  trustworthy. 

"  Campbell  has  found  that  the  velocity  of  the  solar 
motion,  with  reference  to  stars  of  spectral  types  B,  A, 
and  F  to  F4,  inclusive,  is  in  essential  agreement  with 
the  velocity  deduced  from  stars  of  spectral  types  F5 
to  G,  inclusive,  K  and  M. 

"It  does  not  clearly  appear  that  the  direction  and 
speed  of  the  solar  motion  are  functions  of  the  dis- 
tances of  the  stars  used  as  a  basis  for  the  solutions. " 

STAR  GROUPS 

Whether  the  sun  has  companion  stars  in  its  course 
is  not  known  certainly,  but  there  are  known  groups  of 
stars  which  seem  to  form  well-defined  systems  moving 
with  a  common  trend.  Such  a  group  is  the  Pleiades, 
including,  besides  the  six  stars  easily  visible,  a  much 

398 


•     PLATE  XIX 


THE  PLEIADES.     (G.  W.  Ritchey.) 

Photographed  with  the  2-foot  reflector  of  the  Yerkes  Observatory,  1901, 
October  19.     Exposure  3^  hours.     Cramer  Crown  plate. 


..-•,•;• 


THE   SUN   AMONG   THE   STARS 

larger  number  of  telescopic  stars.  Their  connection 
to  form  a  common  system  is  shown  by  at  least  three 
kinds  of  evidence.  First,  it  is  highly  improbable  that 
so  many  stars  of  that  brightness  would  fall  in  so  small 
a  region  of  the  sky,  if  the  distribution  was  purely  at 
random.  Second,  excluding  a  few  stars  not  regarded 
as  belonging  to  the  system,  the  stars  mentioned  have 
equal  proper  motions  in  the  same  direction.  The 
common  proper  motion  is  about  6"  per  century. 
Third,  there  is  a  nebula,  of  filmy  cloud  patch  in  the 
sky,  visibly  connected  with  the  several  stars  of  the 
group  and  evidently  confirming  their  common  con- 
nection (see  Plate  XIX).  The  Pleiades  group,  in- 
cluding small  stars  partaking  of  the  common  motion, 
measures  nearly  100'  of  arc  in  average  diameter.  The 
parallaxes  of  the  stars  are  not  certainly  measurable, 
but  their  distance  has  been  estimated  with  some 
plausibility  to  be  not  less,  at  any  rate,  than  200  light 
years.  Hence,  the  rad  us  of  the  system  is  not  less 
than  three  light  years,  or  18,000,000,000,000  miles, 
which  is  6,000  times  the  radius  of  Neptune's  orbit. 
If,  indeed,  the  group  is  actually  as  small  as  this,  it 
would  mean  perhaps  a  hundred  good  sized  stars 
nearer  together  than  the  sun  is  to  its  nearest  stellar 
neighbor. 

The  curious  connection  of  nebulosity  with  the 
Pleiades  is  not  without  its  counterpart  in  many  other 
regions  of  the  sky,  and  even  our  own  solar  system 
seems  not  to  be  devoid  of  it.  There  is  observable  on 
dark  nights,  nearly  in  the  plane  of  the  ecliptic,  a  light 

399 


THE  SUN 

not  to  be  regarded  as  incipient  twilight,  called  the 
Zodiacal  light  when  viewed  towards  the  sun,  and 
called  the  Gegenschein  in  the  opposite  direction.  See- 
liger  has  estimated,  on  reasonable  assumptions,  that 
it  is  the  matter  contained  in  this  ring  of  nebulosity 
which  causes  the  outstanding  perturbation  in  the 
orbit  of  Mercury,  not  to  be  accounted  for  by  the  at- 
traction of  known  planetary  masses.  It  has  been 
supposed  that  stars  have  their  origin  in  nebulae,  and 
if  so  the  Pleiades  stars  would  seem  to  be  less  advanced 
in  their  course  of  evolution  than  the  sun,  but  we  shall 
recur  to  this. 

Whether  it  is  gravitation  which  controls  the  motion 
of  the  sun  among  the  stars,  and  whether  such  a  vast 
system  as  the  Pleiades  is,  like  the  planetary  systems, 
in  orderly  gravitational  movement,  are  questions 
which  as  yet  there  are  no  means  of  fully  solving,  but 
the  affirmative  is  generally  believed  It  has  been 
computed  by  Newcomb,  however,  that  there  is  not 
enough  matter  in  the  universe  to  control  the  motion 
of  such  runaway  stars  as  1830  Groombridge  and  Arc- 
turus. 

DOUBLE  STARS 

That  gravitation  is  an  universal  property  seems  to 
be  proved  by  the  existence  of  well-observed  elliptical 
orbits  in  the  cases  of  many  pairs  of  double  stars. 
Since  there  are  less  than  ten  thousand  stars  to  the 
sixth  magnitude  in  the  whole  heavens,  the  chances  are 
almost  infinitely  small  that  two  of  them  should  be 
found  within  5"  of  one  another  on  a  random  distribu- 

400 


THE  SUN  AMONG  THE  STARS 

tion,  for  there  are  over  20,000,000,000  squares  of  5" 
in  the  whole  sky.  But  in  fact  there  are  many  pairs 
closer  than  this  among  the  visible  stars,  so  that  a 
physical  connection  in  most  such  cases  is  practically 
certain.  In  some  cases,  as  in  that  of  the  very  bright 
pair  of  0.4  and  1.9  magnitude  composing  a  Centauri, 
stars  separated  by  a  much  greater  interval  (in  this 
case  averaging  17".  1)  are  proved  to  be  physically 
connected  because  they  are  observed  to  go  through  a 
periodic  change  of  position  with  respect  to  one  an- 
other. The  orbit  of  a  Centauri  is  completed  in  eighty- 
one  years.  By  spectroscopic  observations  of  motion 
in  the  line  of  sight  a  great  number  of  stars  not  tel- 
escopically  resolvable  are  proved  to  be  physically 
connected  doubles  because  of  the  variable  velocity 
observed.  In  some  cases  of  spectroscopic  binaries 
the  companions  are  indicated  by  doubling  of  the 
spectrum  lines,  but  quite  often  one  of  the  objects  is 
too  faint  to  give  a  spectrum,  and  its  existence  is  noted 
only  because  the  periodically  variable  positions  of  the 
lines  in  the  observed  spectrum  indicate  that  the  star 
observed  is  affected  by  orbital  motion. 

STELLAR  MASSES  AND  DENSITIES 

The  spectroscopic  method  gives  the  projection  on 
the  line  of  sight  of  the  linear  velocity  of  one  or  of  both 
components  in  their  orbits.  The  telescopic  method 
gives  the  projection  at  right  angles  to  the  line  of  sight 
of  the  angular  motion  of  the  components.  Both 
methods  give  the  period  of  the  revolution.  When  the 

401 


THE  SUN 

parallax  of  the  object  is  known,  as  in  the  case  of  a 
Centauri,  the  projected  linear  dimensions  of  the 
orbits  are  easily  found.  It  is  possible  in  the  case  of 
accurately  observed  telescopic  binaries  of  known  par- 
allax, or  of  pairs  whose  motions  have  also  been  ob- 
served spectroscopically,  to  determine  the  actual 
linear  dimensions  of  the  two  orbits,  and  (assuming  the 
law  of  gravitation)  the  relative  masses  of  the  two 
stars.  The  combined  mass,  as  compared  with  the 
combined  mass  of  the  earth  and  sun,  follows  easily 
from  Kepler's  third  law.  For  if  we  regard  the  mass 
of  the  earth  and  sun  combined  as  unit  mass,  the  rad- 
ius of  the  earth's  orbit  as  unit  distance,  and  the  year 
as  unit  time ;  then  calling  the  period,  total  mass,  and 
mean  radius  vector  of  the  binary,  P,  M,  and  R,  re- 
spectively, we  have,  if  matter  has  the  same  constant 
of  gravitation  everywhere: 

R3 
M- pi. 

Since  R  and  P  are  both  known  for  a  well-determined 
orbit,  we  thus  find  M,  the  ratio  of  the  combined  mass 
of  the  binary  to  the  combined  mass  of  earth  and  sun. 
In  the  case  of  a  Centauri  the  total  mass  is  twice  that 
of  the  sun,  and  the  components  being  approximately 
of  equal  mass,  they  are  singly  about  of  the  same  mass 
as  the  sun.  Their  mean  distance  apart  is  23.6  times 
the  radius  of  the  earth's  orbit. 

By  such  processes  the  combined  masses  of  various 
binary  stellar  systems  have  been  determined.  The 
resulting  masses  are  sometimes  less,  sometimes  a  few 

402 


THE  SUN  AMONG  THE  STARS 

fold  greater  than  the  sun's.  Had  they  come  out  of 
another  order  of  magnitude  entirely,  it  would  have 
seemed  doubtful  if  the  constant  of  gravitation  has  the 
same  value  in  other  systems  than  ours.  But  as 
things  are,  we  seem  justified  in  supposing  that  gravi- 
tation is  an  universal  unchanging  property  of  matter. 
There  is  a  method  proposed  by  Pickering  for  find- 
ing a  relation  between  the  surface  brilliancy  and  den- 
sity of  the  average  star  of  a  binary  system  whose 
period  and  magnitude,  on  the  scale  of  brightness,  are 
known.  Without  going  into  an  explanation  of  the 
matter,  which  may  be  found  in  works  on  the  stars,  we 
shall  be  interested  in  the  conclusion,  which  is  that 
stars  in  general  give  much  more  light  in  proportion  to 
their  masses  than  does  the  sun.  Astronomers  gener- 
ally incline  to  believe  that  the  discrepancy  indicates 
for  the  stars  generally  a  smaller  density  than  that  of 
the  sun.  In  a  few  instances,  another  line  of  argument 
regarding  star  densities  is  possible.  There  are  some 
binary  systems  whose  orbits  are  of  such  small  dimen- 
sions, and  lie  so  nearly  in  a  plane  with  the  earth,  that 
the  components  regularly  eclipse  one  another,  and 
the  quantity  of  light  of  the  binary  thereby  suffers 
periodic  variability.  In  such  a  system  the  duration 
of  the  eclipse  compared  with  the  period  of  the  orbit, 
gives  a  measure  of  the  relative  diameters  of  the  stars 
relatively  to  the  diameter  of  their  orbits.  Proceeding 
in  this  fashion  it  was  shown  by  Roberts  that  the  aver- 
age densities  of  these  variables  (called  "  Algol  vari- 
ables" after  the  name  of  the  famous  spectroscopic 

403 


THE  SUN 

binary  star  which  is  the  type  of  the  class)  is  no  more 
than  one-eighth  that  of  the  sun.  This  general  con- 
clusion was  independently  confirmed  at  the  same 
time  by  Russell. 

We  have  spoken  incidentally  of  the  Algol  type  of 
eclipsing  variable  stars.  Without  going  far  into  the 
discussion  of  stellar  variability,  it  will  be  of  deep  in- 
terest, in  view  of  the  somewhat  irregular  and  very 
slight  variability  of  the  sun,  to  speak  of  another  kind 
of  variable  stars  of  which  Omicron  Ceti  (or  Mira 
Ceti)  is  the  type.  This  star  is  sometimes  as  bright  as 
the  second  magnitude,  and  sometimes  as  faint  as  the 
ninth  or  fainter.  Accordingly  its  range  is  several 
thousandfold  in  brightness.  It  goes  through  its 
cycle  in  an  average  period  of  about  331.6  days,  but  is 
sometimes  thirty  or  forty  days  early  or  late  in  coming 
to  a  maximum.  Its  maxima  and  minima  are  not 
uniformly  bright,  for  sometimes  it  attains  only  the 
fifth  magnitude  at  maximum,  and  sometimes  it  falls 
only  to  the  eighth  magnitude  at  minimum.  The 
time  required  to  rise  from  minimum  to  maximum 
brightness  is  only  about  two-thirds  the  time  required 
to  fall  to  a  minimum.  The  shape  of  the  light  curve  is 
variable  too,  as  the  maxima  continue  longer  at  some 
recurrences  than  at  others. 

The  spectrum  of  Mira  is  of  the  third  type,  to  which 
Antares  belongs/  distinguished  by  the  fluted  spectra 
found  to  some  extent  in  sun  spots.  The  spectrum 
varies  as  the  star's  brightness  varies,  becoming 

1  See  below. 
404 


THE  SUN  AMONG  THE  STARS 

stronger  in  the  violet,  and  especially  in  its  bright 
violet  hydrogen  lines,  in  the  maximum  phases.  The 
spectrum  indicates  a  high  velocity  of  recession  from 
the  sun  (66  kilometers  per  second),  but  there  is  no 
evidence  from  it  that  Mira  has  companions. 

In  many  respects  Mira's  variability  suggests  the 
solar  variability  associated  with  sun  spots.  True,  the 
fractional  change  of  solar  radiation  is  perhaps  not 

more  than  7::  as  great  as  that  of  Mira,  but  in  the 


existence  of  a  fixed  average  period  subject  to  large 
individual  departures,  an  unequal  intensity  of  max- 
ima, an  unsymmetrical  and  variable  light  curve, 
there  is  a  strong  similarity  to  what  the  sun  spot  curve 
suggests  for  the  sun.  In  one  respect  there  is  a  di- 
vergence. Mira  increases  in  brightness  faster  than 
it  decreases.  The  change  of  temperature  of  the 
earth  seems  to  indicate  that  the  sun's  radiation  is  at 
a  maximum  when  sunspots  are  fewest.  But  the  sun 
spots  decrease  to  a  minimum  slower  than  they  in- 
crease to  a  maximum.  Still,  with  so  many  features 
of  similarity  there  can  be  little  doubt  that  the  dis- 
covery of  the  cause  and  accompanying  phenomena 
of  the  sun  spot  periodicity  will  indicate  the  secret 
of  the  Mira  type  of  variables. 

STELLAR  SPECTRA 

Having  taken  some  note  of  the  distances,  motions, 
brightnesses,  masses  and  densities  of  the  stars  as 
compared  with  that  of  the  sun,  and  having  seen  that 

405 


THE  SUN 

we  owe  this  information  to  knowledge  gained  of  the 
sun  itself,  and  of  the  solar  system,  we  may  now  turn 
our  attention  to  the  spectra  of  the  stars,  and  see 
wherein  and  how  far  the  sun  is  a  type  in  that  respect. 
We  have  noted  that  in  the  solar  spectrum  dark  lines 
of  the  metals  are  the  prevailing  feature.  Calcium 
and  hydrogen  lines  sometimes  give  bright  reversals  in 
their  centers.  Helium  seldom  produces  a  .dark  pho- 
tospheric  line,but  in  the  spectrum  of  the  chromosphere 
its  bright  lines  are  conspicuous  along  with  those  of 
hydrogen  and  calcium.  In  the  spectra  of  sun  spots 
the  dark  lines  of  metals  are  still  conspicuous,  but  are 
nearly  overshadowed  in  importance  by  banded  spec- 
tra of  various  compounds,  and  the  violet  end  of  the 
spectrum  is  very  weak  in  them  compared  with  the 
red,  or  with  the  violet  of  the  ordinary  photospheric 
spectrum. 

These  various  peculiarities  of  the  solar  spectra  find 
counterparts  in  the  stars. !  There  is  a  large  class  of 
stars  whose  spectra  are  hardly  to  be  distinguished, 
line  for  line,  from  that  of  the  sun.  Among  the  most 
exact  duplicates  is  the  spectrum  of  the  principal  star 
of  the  brilliant  binary  system  Capella.  From  this 
solar  type  we  can  pass  either  way;  in  the  one  direc- 
tion to  stars  on  which  the  red  predominates,  and 
banded  spectra  overshadow  the  metallic  lines,  or  in 
the  other  direction  to  blue  stars  in  which  lines  of 
hydrogen  or  helium  are"  almost  the  sole  features  aside 
from  the  ^continuous'  spectrum. 

By  the  kindness/of  Director  Campbell  of  the  Lick 
406 


THE  SUN  AMONG  THE  STARS 

Observatory  and  Director  Frost  of  the  Yerkes  Ob- 
servatory, I  give  here  in  Plates  XXA  and  B  and 
XXI  a  series  of  spectra  illustrative  of  the  gradation 
from  the  so-called  helium  or  Orion  stars  to  the  so- 
called  carbon  stars,  which  lie  at  opposite  ends  of  the 
scale.  While  considering  these,  let  us  note  more 
closely  the  diversities  of  stellar  spectra. 

The  Classification  of  Stellar  Spectra. 

Father  Secchi  in  1867  divided  the  spectra  of  stars 
into  four  great  classes.  Class  I  comprises  the  blue 
and  white  stars.  In  their  spectra  dark  lines  of  metal 
are  few  and  feeble,  but  the  dark  hydrogen  lines  are 
well  marked.  This  class  is  the  most  numerous,  and 
includes  among  other  prominent  stars,  Sirius,  Vega, 
and  Procyon.  Class  II  comprises  the  yellow  stars 
whose  spectra  are  filled  with  metallic  lines.  This 
class  includes  the  sun,  also  Capella,  Arcturus,  and 
Aldebaran.  Class  III  comprises  orange  and  red 
stars  whose  spectra  show,  besides  many  dark  metallic 
lines  found  in  the  stars  of  the  second  type,  also  nu- 
merous dark  bands  or  flutings.  These  consist,  like 
the  terrestrial  oxygen  bands,  of  series  starting  with 
well-marked  heads  and  shading  off  from  these  to- 
wards the  red.  These  flutings  are  now  recognized 
to  be  caused  by  oxides  of  titanium  and  other  metals, 
and  by  hydrides  as,  for  example,  of  calcium.  This 
class  includes  Antares  and  Betelgeuse.  Class  IV 
comprises  some  deep  red  stars,  whose  spectra  also 
contain  bands  or  flutings,  but  with  the  shadings 
28  407 


THE  SUN 

toward  the  violet.  These  flutings  are  attributed  to 
carbon  or  its- compounds.  The  stars  of  Class  IV  are- 
all  faint.  The  two  brightest  are  nineteen 'Piscium 
(5.3  mag.)  and  152  Schjellerup  (5.5  mag.). 

The  stellar  classification  of  Secchi  is  still  much 
used  in  general  descriptions,  although  more  detailed 
systems  of  classification  have  been  lately  adopted: 
The  accompanying  Plates  XXA  and  B  and  XXI 
illustrate  some  of  the  differences  between  Secchi's 
types.  It  is  however,  practically  impossible,  with- 
out having  had  personal  handling  of  direct  spectrum 
photographs,  to  note  at  a  glance  the  significant  vari- 
ations in  spectra.  The  spectral  types  of  Secchi 
merge,  of  course,  gradually  together,  so  that  in  some 
cases  one  would  be  doubtful  in  which  of  two  classes 
to  assign  a  star.  *• 

There  are  two  principal  modifications  to  be  made 
to  Secchi's  classification.  Fdrst,  and  most  impor- 
tant, among  the  blue  or  white  stars  occur  many  whose 
spectra  are  distinguished  by  the  absorption  lines  of 
helium,  more  than  by  those  of  hydrogen.  Lines 
of  oxygen  and  silicon  also  sometimes  occur  in  these 
helium  star  spectra,  but  most  metallic  lines  are 
extremely  faint  or  invisible.  Helium  stars  are  nu- 
merous in  the  constellation  Orion  and  in  the  Milky 
Way.  'Secchi's  Glass  I  may  then  be  divided  into 
two  principal '  sub-classes,  the  helium  or  Orion  stars, 
and  the  hydrogen  or  Sirian  stars.  The  helium  stars 
not  infrequently  show  some  bright  emission  lines  in 
their  spectra  besides  the  dark  or  absorption  lines. 

408 


c-i 

M 
S 


I 


SESS 


t    H«* 

53         O>    01    0>    0) 

o    sags 


.2.2.2.2 


a  5  C  « 

IPQOO 

>     ti     «     0 


• 


THE  SUN  AMONG   THE  STARS 

Such  bright  lines  are  generally  of  hydrogen,  but  the 
helium  line  D3  is  also  bright.  Vogel's  classification 
includes  a  third  division  of  Class  I  for  such  bright 
line  stars. 

But  there  is  a  class  of  stars  for  which  Pickering  at 
one  time  proposed  to  add  a  Class  V  to  Secchi's  sys- 
tem, whose  spectra  have  as  their  main  characteristics 
bright  lines  or  bands  in  the  yellow  and  blue,  some  due 
to  hydrogen,  others  of  unknown  origin.  The  bright 
line,  or  so-called  Wolf-Rayet  stars,  are  situated 
mostly  in  the  Milky  Way  or  the  Magellanic  Clouds, 
and,  except  7  Vela,  are  faint  stars.  Some  of  the 
ultra-violet  lines  bright  in  the  spectra  of  Wolf-Rayet 
stars  are  also  bright  lines  in  the  spectra  of  certain 
nebulae. 

There  has  been  adopted  at  the  Harvard  College 
Observatory  a  more  detailed  system  of  stellar  classi- 
fication than  either  Secchi's  or  Vogel's,  and  which 
includes  numbered  gradations  of  the  lettered  main 
divisions,  so  that  a  very  large  number  of  varieties 
of  spectra  may  be  indicated.  A  spectrum  marked 
B3A,  or  more  briefly  B3,  is  one  which  is  estimated  to 
be  three-tenths  the  way  from  a  typical  B  star  to  a 
typical  A  star,  and  similarly  for  other  combinations. 
The  following  table  gives  parallel  designations  of 
typical  stars  under  the  classifications  of  Secchi,  Vogel, 
and  Harvard  College  Observatory.  In  the  Harvard 
classification  the  type  Q  is  reserved  for  the  "new 
stars"  which  have  passed  their  paroxysm  of  bright- 
ness. Oa  is  the  designation  of  Wolf-Rayet  stars. 

409 


THE  SUN 


Class  0  has  other  subscripts  b,  c,  d,  and  class  M  has 
subscripts  b,  c,  not  included  in  the  table. 

TABLE  XXXI.— -Classification  of  stellar  spectra 


STAR 

Harvard 

Vogel 

Secchi 

(77  Argus,  or  Carina)  
(7  Argus,  or  Vela).  
(29  Canis  Majoris)  

A 

Oe 

Ib 
Ib 

- 

(A  Orionis). 

Oe5B 

Ib 

I 

(5  Orionis)  
Alcyone       (77  Tauri) 

B 
B5A 

Ib 
Ib 

I 
I 

Sirius            (a  Canis  Majoris)  
Altair           (a  Aquilse)  
Canopus      (a  Argus,  or  Carina)  
Procyon       (a  Canis  JMinOris)  
The  Sun       Also  Capella  (a  Auriga)  
(K  Geminorum)  
Arcturus       (a  Bootis)  
Aldebaran    (a  Tauri)  .  .-  
Betelgeuse   (a  Orionis)  
Mira            (o  Ceti)  .  
(19Piscium)  

A 
ASF 
F 
F5G 
G 
G5K 
K 
K5M 
Ma 
Md 
N 

la 
Ia3-IIa 
Ia3-Ha 
Ia3-Ha 
11. 
Ila-HIa 
I1.-HI. 
lIa-IIIa 
IHa 
IHa 
IHb 

I 
I 
I 
I 
II 
II 
II 
II 
III 
III 

IV 

In  some  stars  the  spectrum  of  hydrogen  assumes 
form  which  was,  to  be  sure,  predicted  from  the  nu- 
merical spectrum  series  relations,  but  which  has  never 
been  experimentally  produced  in  the  laboratory.  We 
cannot  yet  tell,  therefore,  what  conditions  such  stars 
typify.  The  striking  analogy  between  the  third  type 
spectra  and  those  of  sun  spots,  taken  in  connection 
with  the  proved  relatively  low  temperature  of  sun 
spots  noted  in  Chapter  IV,  indicate  clearly  a  pro- 
gression of  temperature  from  stars  of  type  II  to  those 
of  type  III  as  well  as  of  spectrum. 

SPECTRAL  DISTRIBUTION  OF  RADIATION 
Wilsing  and  Scheiner  have  lately  made  a  long 
series  of  spectral  photometric  observations  on  stars 

410 


THE   SUN   AMONG   THE   STARS 


of  Vogel's  various  spectral  types,  to  see  how  the  dis- 
tribution of  energy  in  their  spectra  compares ;  and  with 
a  view  to  estimating  the  temperatures  prevailing  in  the 
stars  by  comparison  with  the  distribution  computed 
for  " black-body"  spectra.  Their  conclusion  is  that 
the  temperatures  vary  regularly  with  the  type  among 
the  stars  investigated,  from  upwards  of  10,000°  of 
the  absolute  centigrade  scale,  to  below  3,000°.  As 
there  is  some  question  as  to  the  validity  of  the  tem- 
peratures deduced,  I  give  here  what  seems  a  more 
direct  and  quite  as  interesting  a  summary  of  their 
results,  namely,  the  mean  spectral  distributions  for 
four  average  stars  of  each  of  the  seven  spectral 
classes  investigated.  The  spectra  have  been  put 
equal  at  wave  length  0.448/-6. 

TABLE  XXXII . — Intensities  in  stellar  spectra.    (Wilsing  and  Scheiner.) 


INTENSITY 

STARS 

A0?448 

A0?480 

A0?584 

A0^638 

»'  { 

0  Can.  Afin.,      12  Can.  Yen. 
aDelphini,           aPegasi 

JlOOO 

836 

579 

505 

Ia2  { 

aAndrom.,          y  Coronae 
y  Ophiuchi,          y  Lyrae 

}  1000 

796 

625 

525 

Ia3-IIa..   { 

a.  Trianguli,         £  Geminorum 
8  Leonis,               8  Aquilae 

}  1000 

948 

902 

845 

»:.  ..{ 

y  Pegasi,              t\  Leonis 
p  Leonis  ,               £  Pegasi 

}  1000 

887 

578 

530 

Ila  J 

17  Bootis,               fi  Virginis 
ju.  Herculis,           y  Cygni 

J  1000 

998 

993 

1005 

Ila-IIIa.  | 

a.  Arietis,              o  Tauri 
S  Cancri,              ft  Ophiuchi 

|  1000 

1205 

1766 

1897 

Ilia  | 

a  Orionis,             8  Virginis 
X  Serpentis,           8  Sagittac 

|  1000 

1368 

3296 

4406 

According  to  the  energy  spectrum  data  given  in 
Chapter  III  the  sun's  spectrum  would  fall  in  their 

411 


4 


THE  SUN 

class  (Ia3-IIa).  The  results  given  indicate  that  the 
order  of  the  series  of  spectra,  given  by  Vogel  from  an 
inspection  of  the  character  of  the  Fraunhofer  lines, 
has  a  strong  support  from  the  distribution  of  inten- 
sities of  the  continuous  spectra  as  well.  Furthermore, 
the  order  given  is  the  order  proper  to  a  series  of 
spectra  from  sources  of  successively  lower  and  lower 
temperatures. 

EVOLUTION  OF  THE  SOLAR  SYSTEM 

The  inquiring  mind  is  ever  stimulated  by  the 
query:  What  means  the  order  of  the  heavens,  and 
can  we  not  draw  from  it  a  reasonable  view  of  the 
evolution  of  the  universe,  including  the  solar  system? 
The  famous  Laplace,  in  1796,  crystallized  and  ampli- 
fied the  conceptions  which  earlier  philosophers 
had  foreshadowed  into  his  famous  Nebular  Hypoth- 
esis of  the  formation  of  the  solar  system.  As  mod- 
ified a  half  century  later  by  the  discovery  of  the  con- 
servation of  energy,  it  presumes  a  gaseous  nebula, 
larger  than  Neptune's  orbit,  in  primitive  rotation, 
By  virtue  of  its  immense  extent  it  contains  potential 
energy  of  position  which  is  transformed  into  heat  as 
the  nebula  condenses,  and  thus  is  supplied  the  energy 
of  radiation.  The  gravitation  of  the  nebula  in  con- 
junction with  the  occasional  collisions  of  its  mole- 
cules tended,  it  is  supposed,  to  produce  condensation. 
At  certain  critical  times  the  revolving  mass  separated 
rings,  and  these  by  condensation  produced  the  plan- 
ets. The  planets  in  condensation  likewise  threw  off 


THE  SUN   AMONG   THE   STARS 

rings  which  formed  the  moons.  In  Saturn's  case 
rings  still  persist.  The  view  accounts  for  the  pre- 
vailing tendency  of  the  planets,  their  satellites,  and 
the  sun,  to  rotate  in  the  same  direction,  and  for  the 
approximately  common  plane  of  their  orbits  and  ro- 
tations. The  exceptions  of  retrograde  motion  were 
not  known  in  1796,  nor  were  they  discussed,  so  far 
as  is  known,  by  Laplace,  in  his  later  revisions  of  his 
theory. 

According  to  Chamberlin  and  Moult  on  the  La- 
placian  hypothesis,  even  as  modified  and  clarified  by 
the  work  of  Helmholtz,  Roche,  Darwin,  and  others, 
fails  conspicuously  to  account  for  a  number  of  things. 
Principal  among  these  are:  A.  The  considerable 
eccentricities  of  some  of  the  planetary  orbits  and  the 
inclinations  of  their  planes  among  themselves,  and 
with  respect  to  the  sun's  equator.  B.  The  neg- 
ative rotation  of  some  of  the  satellites,  and  the  small 
periods  of  revolution  of  some  of  them  as  compared 
with  the  periods  of  rotation  of  their  primaries.  C. 
The  difficulty  of  understanding  how  rings  could  be 
left  off  in  the  shrinking  of  the  nebula,  whether  it 
were  gaseous  or  meteoric  in  structure,  and  the  still 
greater  difficulty  of  understanding  how  a  ring,  if  left 
off,  could  condense  into  a  planet.  D.  The  difficulty 
of  accounting  for  the  enormous  discrepancy  between 
the  present  moment  of  momentum  of  the  system 
and  that  which  must  apparently  have  formerly  pre- 
vailed. 

Chamberlin  and  Moulton  have  proposed  the 
413 


THE  SUN 

" Planetesimal  Hypothesis"  of  the  evolution  of  the 
solar  system.  They  might  start  with  a  spiral  nebula. 
Since  there  are  millions  of  these  objects  in  the  sky 
this  basis  is  justified.  But  the  authors  have  gone 
even  further,  and  suggested  that  in  the  course  of 
ages  two  stars  may  approach  so  near  together  that 
they  will  mutually  raise  enormous  tides.  Tides 
occur  in  pairs  at  opposite  ends  of  a  diameter.  Such 
tremendous  disturbances  as  thus  supposed,  together 
with  the  eruptive  tendencies  due  to  intense  heat, 
would  perhaps  combine  to  cause  many  masses  of 
matter,  varying  greatly  as  to  quantity,  to  be  pro- 
jected from  each  tidal  region.  The  relative  motion 
and  gravitation  of  the  two  stars  would  tend  to  change 
the  motion  of  projection  of  the  masses  into  motion 
of  revolution  in  orbits  about  the  primaries.  When 
the  action  first  occurred,  the  disturbing  star  being  far 
off,  and  the  attraction  of  the  erupting  star  acting 
preponderatingly,  the  orbits  of  the  erupted  masses 
would  be  small,  and  their  periods  of  rotation  short. 
At  closest  approach  of  the  disturbing  star  the  con- 
trary would  prevail.  The  outcome  would  be  a  two- 
branched  spiral  (see  Plate  XXV),  containing  many 
masses  of  all  sizes  revolving  in  orbits  about  the  parent 
star  (our  sun).  As  the  inner  orbits  are  of  less  period 
than  the  outer,  the  spiral  form  will  become  more  and 
more  coiled,  and  at  length  cease  to  present  a  spiral 
appearance. 

Mutual  attraction  and  collisions  among  the  numer- 
ous masses  would  lead  to  the  concentration  of  the 

414 


THE  SUN  AMONG  THE  STARS 

lesser  masses  and  particles  on  the  larger  ones,  or 
their  revolution  about  them  as  satellites.  The  au- 
thors show  that  collisions  tend  on  the  whole  to  de- 
crease the  ellipticity  of  the  supposed  orbits,  so  that 
the  larger  planets,  on  which  most  collisions  have 
occurred,  would  have  the  most  nearly  circular  orbits. 
The  lighter  gases  would  be  early  lost  as  atmospheres 
from  the  smaller  planets  and  satellites  owing  to  the 
consequences  of  the  kinetic  theory  of  gases.  But 
with  increasing  size  caused  by  the  accretion  of  par- 
ticles in  collision,  the  occluded  gases  would  be  forced 
out  of  the  interior  by  growing  pressure,  and  so  after 
a  time  atmospheres  would  be  supplied  again  to  plan- 
ets of  medium  size.  The  large  planets  would  retain 
the  gases  from  the  start  as  atmospheres. 

The  numerous  fragments  called  the  asteroids  re- 
mained almost  unaltered  from  lack  of  large  masses  in 
their  neighborhood  to  capture  them.  Their  eccen- 
tric orbits  and  high  inclinations  are  evidence  of  the 
comparative  rarity  of  collisions  among  them.  The 
retrograde  motions  and  relatively  high  velocities 
occurring  among  the  satellites  seem  to  present  no 
difficulty  in  the  view  of  the  authors. 

The  plane  of  the  sun's  rotation  they  believe  to 
have  been  modified  by  the  falling  back  of  much 
ejected  material  not  forced  into  clear  orbits.  Prob- 
ably the  original  plane  of  rotation  was  at  consider- 
able angle  to  the  present,  but  has  been  brought 
nearer  the  average  plane  of  the  planetary  orbits  by 
such  collisions, 

415 


THE  SUN 

Perhaps  the  greatest  difficulty  of  the  hypothesis 
is  to  account  for  the  supply  of  solar  radiation  during 
the  immense  period  of  time  that  the  earth,  as  shown 
by  geological  evidence,  has  retained  practically  its 
present  dimensions  and  form,  and  its  present  tem- 
perature. Moulton  assumes,  as  the  only  explanation 
available,  and  one  which  he  thinks  is  also  required 
by  the  Laplacian  theory,  that  probably  the  contrac- 
tion theory  of  the  sun's  heat  accounts  for  only  a 
small  part  of  the  solar  energy.  It  would  seem  as  if 
the  Laplacian  theory  had  a  great  advantage  here,  for 
it  presupposes  the  general  extension  of  the  nebula 
beyond  the  orbit  of  the  earth,  when  the  earth  began 
to  form.  Hence  there  was  an  immense  store  of  en- 
ergy to  be  gained  by  contraction.  On  the  contrary, 
the  spiral  nebula  of  Chamber lin  and  Moulton  appar- 
ently had  no  such  general  extension,  but  retained 
nearly  all  of  its  matter  from  and  after  the  catastrophe 
in  the  center  of  things.  Furthermore,  the  general 
extension  of  the  nebula  of  Laplace  enables  us  to  sup- 
pose that  the  earth  was  for  a  very  long  time  receiving 
radiation  from  a  large  portion  of  a  hemisphere,  or 
even  (by  reflection  within  vestiges  of  the  nebula) 
from  a  sphere,  so  that  we  need  not  suppose  that  the 
intensity  of  this  radiation  was  great,  and  therefore 
we  can  assign  a  very  much  longer  life  to  the  contrac- 
tion source  of  energy  than  we  could  if  we  were  obliged 
to  think  of  the  solar  radiation  as  always  requiring 
to  be  at  its  present  intensity,  during  geologic  time, 
in  order  to  maintain  terrestrial  temperature. 

416 


THE  SUN  AMONG  THE  STARS 

Prof.  T.  J.  J.  See  has  just  published  (after  the  above 
resume  of  nebular  hypotheses  was  written)  his  vol- 
ume on  this  subject.  In  his  view  the  sufficiently 
close  approach  of  two  stars  to  form  a  spiral  nebula, 
as  assumed  by  Chamberlin  and  Moulton,  is  too  in- 
frequent to  deserve  consideration.  He  would  assume 
the  spiral  nebulae  to  be  formed  by  the  close  approach 
of  two  nebulous  streams,  and  the  curling  of  them 
together  by  mutual  gravitation,  or  by  the  curling  up 
of  a  single  nebulous  stream  owing  to  its  own  grav- 
itation, but  he  does  not  show  that  such  phenomena 
are  apt  to  happen  more  frequently  than  that  sug- 
gested by  Chamberlin. 

Such  a  spiral  nebula  is,  according  to  him,  the  parent 
of  the  solar  system,  but  unlike  Chamberlin  and 
Moulton,  his  nebula  would  not  have  its  central  con- 
densation, the  sun,  mainly  formed  before  the  planets 
began  to  form,  but  all  would  be  forming  at  the  same 
time,  by  capture  of  particles  by  larger  masses  in  the 
exercise  of  mutual  gravitation,  and  in  the  vicissitudes 
of  mutual  encounter  between  the  larger  and  smaller 
bodies  of  the  nebula.  There  seems  to  be  much  in 
common  between  this  "  capture  theory"  and  Moulton 
and  Chamberlin's  accretion  theories.  See  finds  that 
the  orbits  of  the  planets  will  be  rounded  up  by  the 
resistance  (that  is,  the  continual  encounter  with  par- 
ticles) which  they  find  in  the  nebulous  medium.  Here 
he  is  in  close  accord  with  Chamberlin  and  Moulton, 
who  have  found,  as  stated  above,  "that  collisions  tend 
on  the  whole  to  decrease  the  ellipticity  of  the  supposed 

417 


THE  SUN 

orbits,  so  that  the  larger  planets,  on  which  most  col- 
lisions have  occurred,  would  have  the  most  nearly 
circular  orbits."  But  See  believes  further  that  the 
present  orbits  of  the  planets  are  very  far  within  the 
orbits  they  had  when  they  were  principally  formed. 
It  would  seem  on  the  whole,  that,  excepting  in  the 
method  of  forming  his  nebula,  Professor  See's  views 
follow  the  general  lines  laid  down  by  Chamberliri 
and  Moulton,  but  with  this  difference  that  they 
allow  the  sun  to  be  forming  at  the  same  time  as  the 
planets,  and  in  a  very  extended  space,  so  that  the 
problem  of  supplying  energy  by  contraction  for  con- 
tinuing the  solar  radiation  in  ample  measure  through- 
out geological  time  is  easier  for  See  than  for  Chamber- 
lin  and  Moulton.  See's  -conception  also  permits  us 
to  suppose  the  solar  part  of  the  nebula  was  so  much 
expanded  as  to  shine  upon  the  earth  from  a  large 
angle  in  the  earlier  geological  epochs,  as  was  required 
for  the  foundation  of  what  we  have  termed  "  Hypo- 
thesis (B) "  in  Chapters  VI  and  VII. 

STELLAR  EVOLUTION 

We  will  now  consider  a  little  more  closely  the  gen- 
eral view  that  nebulae  are  stars  in  the  making,  and 
that  the  stars  progress  through  a  series  of  tempera- 
tures, and  at  length,  like  the  earth  and  moon,  reach  a 
cold  final  condition.  Plates  XXII  to  XXVI  give  a 
series1  of  nebulous  forms  ranging  from  the  chaotic 

1  It  is  very  questionable  if  we  should  interpret  this  series  of  forms 
as  implying  a  series  in  order  of  development.  I  am  greatly  indebted 
to  my  friend,  Mr.  G.  W.  Ritchey,  for  this  fine  group  of  photographs. 

418 


THE  GREAT  NEBULA  IN  ORION.     (G.  W.  Ritchey.) 

Photographed  with  the  2-foot  reflector  of  the  Yerkes'   Observatory, 
1901,  October  19.     Exposure  1  hour.     Cramer  Crown  plate. 


THE  SUN  AMONG  THE  STARS 

nebulae  in  Orion  and  Cygnus  to  the  well-developed 
spiral  and  ring  forms  of  Andromeda  and  Lyra. 
The  number  of  nebulae  observable  with  the  Mount 
Wilson  reflector  probably  reaches  into  the  millions. 
In  Plate  XIX  we  saw  that  the  Pleiades  stars  are 
plainly  wrapped  in  nebulosity,  and  seem  as  if  still  in 
process  of  condensation.  This  peculiarity  is  shared 
by  other  star  groups,  notably  by  some  in  Orion. 
The  spectra  of  the  Orion  stars  have  a  simplicity  far 
more  in  common  with  the  simple  spectra  of  gaseous 
nebulae  than  with  the  lined  and  banded  spectra  of 
the  solar  and  Antarian  stars.  Stars  of  the  Orion 
type  have  in  many  instances  nebulous  appendages, 
and  besides  seem  to  be  of  extremely  small  density, 
according  to  the  tests  we  have  noted  above.  Hence, 
it  is  supposed  that  the  first  evolutionary  step  is  the 
passage  from  a  nebula  to  a  helium  star.  Nevertheless, 
it  is  found  that  the  great  spiral  Andromeda  nebula 
gives  at  its  center  an  essentially  solar  type  of  spec- 
trum. 

But  even  admitting  the  connection  of  cloudlike 
nebulae  and  helium  stars,  why  should  we  believe  that 
the  nebula  is  the  first  and  not  the  last  end  of  the 
chain,  in  point  of  time,  or  that  the  other  types  of 
spectrum  have  the  same  order  in  their  secular  de- 
velopment as  they  do  in  our  arrangement  of  them 
according  to  their  physical  appearance?  As  to  the 
first  branch  of  the  question,  we  know  that  gravita- 
tion tends  to  condense  matter,  whether  by  capture  as 
of  the  meteors  by  the  earth,  by  the  opportunities 

419 


THE  SUN 

offered  in  molecular  collisions,  as  required  by  La- 
place's Nebular  Hypothesis,  or  by  .the  collision  of 
meteors  in  orbits,  as  proposed  by  Chamberlin  and  by 
See.  In  any  of  these  cases  the  centrally  directed 
force  of  gravitation  inevitably  seizes  its  opportunity 
to  draw  in  the  retarded  particle.  Excepting"  the  dis- 
ruptive tendency  of  the  close  approach  of -two  stars 
invoked  by  Chamberlin  and  Moulton,  and' the  escape 
of  gases  by  molecular  activity  according  to  John- 
stone  Stoney,  there  is  not  known  any  cause  for  the 
separation  of  the  constituents  of  a  star  into  a  nebula. 
This  would  require  an  enormous  expenditure  of  en- 
ergy, whose  possible  source,  except  as  just  indicated, 
it  is  hard  to  conceive.  The  probability  of  the  close 
approach. of  two  stars  would  seem  at" first  sight  to  be 
very  small;,  for.Newcomb  has  computed  that  on  the 
average  a  sphere  of  radius  41 2y 500  times  the  radius 
of  the  earth's  orbit  contains  but  one  visible  star;  On 
the  other  hand,  there  may  be  enormous  numbers  of 
invisible  bodies,  and  even  the  number  of  stars  in 
space  is  so  large  that  such  near  collisions  may  actually 
occur  rather  frequently, .measuring  time  by  centuries. 
We  shall  recur  to  the  question  of  the  order  of  events 
in  stellar  evolution. 

Admitting  the  view  that  nebulae  generally  tend  to 
condense,  not  to  expand,  tKeir  rise  of  temperature 
with  condensation,  if  gaseous,  was  proved  by  Lane  in 
1876.  If  we  adopt  the  usual  view  that  yellow  stars 
are  more  advanced  than  the  blue  ones,  how  are  we 
to  explain  the  circumstance  that  the  blue  stars,  which 

420 


xxni. 


NEBULA  N.  G.  C.  6992  CYGNI.     (G.  W.  Ritchey.) 
Photographed  with  the  2-foot  reflector  of  the  Yerkes'  Observatory, 
1901,  October  5.     Exposure  3  hours.     Cramer  Crown  plate. 


THE  SUN  AMONG  THE  STARS 

seem  to  be  nearest  the  nebulae  in  type,  are  by  Wilsing 
and  Schemer's  observations,  and  by  an  appeal  to 
ordinary  experience,  apparently  hotter  than  the 
yellow  ones?  As  a  reply  to  this  objection  we  must 
note  that  whether  with  most  astronomers  we  accept 
a  photosphere,  or  assume  a  purely  gaseous  sun  as 
discussed  in  Chapter  VI,  the  inner  parts  of  the  sun, 
or  of  a  star,  are  not  visible  to  the  observer.  The  inner 
parts  may  in 'fact  be  hotter  for  yellow  than  for  blue 
stars,  without  in  any  way  altering  the  succession  of 
apparent  surface  temperatures  found  by  Wilsing  and 
Scheiner,  for  different  type  stars. 

But  it  is  by  no  means  clear  that  a  yellow  star  is 
necessarily  older  than  a  blue  star  in  actual  time,  and 
indeed  it  does  not  seem  necessary  to  admit  that 
every  star  of  the  helium  or  hydrogen  type  of  spec- 
trum will  necessarily,  with  lapse  of  time,  become  a 
solar  or  Antarian  star.  The  similarity  of  spectrum 
lines  proves  that  certain  elements  found  in  the  earth 
exist  in  the  sun  and  in  the  stars.  When  stars  fail  to 
exhibit  any  of  the  spectral  lines  of  an  element  we  can- 
not know  that  this  element  exists  in  those  particular 
stars,  for  we  are  not  fully  justified  in  supposing  that 
it  does  so  on  the  assumption  that  conditions  do  not 
favor  the  production  of  its  spectrum.  It  may  pos- 
sibly be,  then,  that  Sirius,  for  instance,  will  never 
show  a  solar  type  of  spectrum,  however  cold  it  may 
grow  superficially. 

I  develop  this  line  of  thought  for  consideration  in 
connection  with  the  discussion  and  catalogue  of 

421 


THE  SUN 


spectroscopic  binaries  recently  published  by  Prof. 
W.  W.  Campbell.1  The  observed  spectroscopic 
binary  stars  range  in  period  from  less  than  a  day  to 
more  than  a  year,  and  visual  binaries  carry  the  range 
of  periodic  times  up  to  thousands  of  years  at  least. 
Campbell,  in  summarizing  the  existing  observations 
of  spectroscopic  binaries,  draws  attention  to  certain 
relations  between  the  periods  of  orbital  revolution, 
eccentricities  of  orbits,  and  types  of  spectrum.  The 
following  table  shows  these  results: 

TABLE  XXXIII. — Spectroscopic  binaries.    Spectral  types,  periods,  and 
eccentricities*  * 


Periods    ~*  
O  and  B  Type*  

Mean  Period  

"short" 

s 

short 

Od-5d 
15 

2d4 

5d-10d 

10 

6d9 

10d 
14 

73d2 

years  ,_ 

1  .!! 

"long" 
long 

Mean  Eccentricity  

(10)0.04 

(5)0.10 

(11)0.34 

(1)0.0 

-    A  Types  
Mean  Period.  

4 
short 

10 
2d65 

9d2 

12  - 
42d2 

2 
26.45 

Mean  Eccentricity  

(5)  0  .  04 

(1)0.50 

(8)  0  .  55 

(1)0.59 

:    F  Types  ! 
Mean  Period  .......... 

0 

G 
3d! 

2 
5d6 

4 
145dl 

3 

11.1    . 

1 

long 

-  Mean  Eccentricity  

(4)  0  .  05 

(1)0.01 

(3)  0  .  15 

(3)0.44 

G  to  M  Types  
Mean  Period 

0 

0 

0 

3 
104dS 

9 
24.3 

13 
long 

Mean  Eccentricity  
Total  

12 

31 

13 

(2)0.06 
33 

(8)0.38 
15 

14 

Mean  Period  

short 

2d59 

6d90 

73d5 

20.5 

long 

Mean  Eccentricity  

(19)0'.  04 

(7)0.14 

(24)0.36 

(13)0.38 

*  From  Lick  Observatory  Bulletin  No.  181. 

This  summary  shows  clearly  that  the  " earlier" 
types  of  spectra  are  associated  in  spectroscopic  binary 

'Lick  Observatory  Bulletin   No.    181.     Also   "Pub.   Astr.   Soc. 
Pacific,"  April,  1910. 

422 


THE  GREAT  NEBULA  IN  ANDROMEDA.     (G.  W.  Ritchey.) 

Photographed  with  the  2-foot  reflector  of  the  Yerkes'   Observatory.  1901, 

September  18.     Exposure  4  hours.     Cramer  Crown  plate. 


THE  SUN  AMONG   THE  STARS 


stars,  as  a  rule,  with  shorter  periods  arid  smaller  ec- 
centricities than  are  the  " later"  types  of  spectra. 
Campbell  also  gives  a  table  of  fifty  telescopic  binaries 
arranged  in  order  of  their  periods  in  five  groups  of  ten 
each.  The  periods  range  from  5.7  years  to  194.0 
years,  and  not  one  of  the  stars  named  (which  gener- 
ally is  the  principal  star  of  the  pair)  has  a  spectrum 
of  the  0  or  B  type,  while  many  specimens  of  types 
A,  F,  and  G,  and  some  of  K,  are  found.  As  for  the 
eccentricities,  these  are  all  large,  averaging  0.461, 
0.453,  0.495,  0.531,  and  0.483  in  the  five  groups. 
The  general  average  period  is  seventy-two  years,  and 
average  eccentricity  of  orbit  0.49. 

In  summary  for  the  telescopic  binaries: 


SPECTRAL 
TYPE 

No.  of 
Stars 

No.  of 
Stars 

Mean 
period 

Mean 
eccentricity 

0-B.. 
A 

0 

9 

Short  periods  

25 

32.8 

0.48 

F 

18 

Long  periods  .... 

25 

108.1 

0.51 

G-K  

M-N  
Unknown.  .  . 

14 
0 
9 

In  the  words  of  Campbell:  " Visual  double  stars 
clearly  abhor  the  O  and  B  types,  and  visual  double 
stars  of  relatively  short  periods  clearly  abhor  M  and 
N  types. 

"What,"    says  Campbell,  "is  the  significance  of 

these  facts?     Let  us  recall  that  Darwin  and  Poincare 

studied  the  origin  of  binary  stars  from  theoretical 

considerations,  and  came  to  the  conclusion  that  a 

29  423 


THE  SUN 

condensing  nebulous  mass,  rotating  on  its  axis  con- 
,  stantly  faster  and  faster,  to  keep  pace  with  loss  of 
heat  by  radiation,  should  eventually  separate  into 
two  nebulous  masses  revolving  around  their  mutual 
center  of  mass.  These  two  masses  would,  in  the 
beginning,  be  revolving  in  contact  in  orbits  essen- 
tially circular.  With  advancing  time,  tidal  disturb- 
ances should  cause  the  two  bodies  to  draw  apart  rap- 
idly at  first,  and  less  rapidly  later.  In  the  spectro- 
scopic  binary  systems  described — have  we  not  a  tol- 
erably complete  sequence  of  orbits  illustrative  of  the 
Darwin-Poincare  hypothesis?  The  short-period  or- 
bits should  be  circular  or  nearly  so,  and  should 
appertain  preferentially  to  stars  of  early  spectral 
types;  the  longer  periods  should,  in  general,  attach 
.to  the  more  eccentric  orbits  and  the  older  spectral 
types;  and  these  are  the  facts  established  by  actual 
observation  of  binary  systems.  ...  It  will  be  noted 
that  in  these  widely  separated  (telescopic  binary)  sys- 
tems there  is  not  a  single  0  or  B  type,  representing  the 
early  stages  of  binary  existence.  There  are  a  few  A 
types,  but  the  major  number  are  of  the  advanced  F 
:type  and  G  and  K  types.  I  suspect  the  K,  M,  and  N 
types  are  not  more  fully  represented  for  the  reason 
'that  in  these  old-age  systems  the  two  components 
are  in  general  so  far  apart  that  the  periods  of  revolu- 
tion are  many  hundreds  or  thousands  of  years." 

Campbell  considers  also  the  relative  masses  of  the 
two  components  in  the  binary  systems  for  which  this 
is  known.  In  seventeen  cases  where  the  components 

424 


o    J,  '  PLATE  XXV. 


SPIRAL  NEBULA.  M.  51  CANUM  VENATICORUM.     (G.  W.  Ritchey.) 

Photographed  with  the  5-foot  reflector  of  the  Mount  Wilson  Solar 

Observatory.     Exposure  10%  hours.     Seed  23  plate. 


THE  SUN  AMONG  THE  STARS 

are  of  unequal  masses  he  finds  that  with  one  excep- 
tion the  lesser  component  is  of  an  "earlier  type  of 
spectrum,  or  bluer,  than  the  more  massive  one." 
Here,  at  first  sight,  is  a  most  surprising  thing.  Of  two 
stars  admittedly  of  equal  age,  the  one  of  greater  mass 
is  in  general  the  more  advanced.  Campbell  says: 
"An  hypothesis  of  Huggins,  suggested  at  first 
rather  casually,  and  later  discussed  more  seriously, 
appears  to  me  to  be  of  great  merit,  especially  when 
Schuster's  extension  of  the  hypothesis  is  applied. 
Huggins'  suggestion  is  as  follows:  'Another  way  of 
looking  at  the  problem  is  perhaps  possible.  May  it 
be  that  the  effect  of  the  great  mass  on  surface  density, 
together  with  the  working  of  Lane's  law,  by  which  the 
temperature  of  a  condensing  gaseous  mass  so  long  as 
it  is  subject  to  the  laws  of  a  purely  gaseous  body  will 
continue  to  rise,  will  favor  in  such  stars  the  coming 
in  of  a  solar  type  of  spectrum  at  a  somewhat  rela- 
tively earlier  time?'  Schuster's  extension  suggests 
in  effect  that  the  lighter  gases — hydrogen,  helium, 
and  so  on — which  surround  a  star  in  its  early  age,  will 
be  pulled  down  on  a  star  of  small  mass  but  lightly, 
and  a  long  period  will  be  required  for  the  absorption 
of  these  gases.  Such  a  star  would  remain  effectively 
young,  as  judged  by  its  spectral  type,  longer  than  its 
more  massive  primary.  In  the  latter,  the  greater 
gravitational  power  would  lead  to  more  rapid  absorp- 
tion of  the  lighter  surrounding  gases,  and  the  pre- 
dominant influence  of  the  metallic  absorption  would 
enter  earlier.  It  seems  reasonable  to  suppose  that 

425 


THE  SUN 

the  greater  internal  gravitation  of  the  more  massive 
primary  will  generate  heat  more  rapidly,  and  cause  it 
to  live  its  life  more  rapidly  than  in  the  case  of  the  less 
massive  secondary." 

Should  not  this  explanation  of  the  prevailing  ten- 
dency to  earlier  types  of  spectra  for  the  lesser  com- 
ponents of  binary  stars  take  more  explicitly  into  con- 
sideration a  difficulty  suggested  by  ordinary  experi- 
ence of  cooling  bodies,  Planck's  law  of  spectral  energy 
distribution,  and  Wilsing  and  Schemer's  results  on 
the  relative  temperatures  of  the  stars?  For  it  is  the 
blue  stars  which  we  should  suppose  to  be,  and  which 
Wilsing  and  Scheiner  find  to  be,  superficially  hottest, 
and  as  the  bluer  stars  are  generally  supposed  to  be 
also  of  less  density  than  the  yellow  ones,  their  sur- 
faces are  also  greater  in  proportion  to  their  masses. 
Hence,  if  their  radiating  coefficients  are  equal  to  those 
of  yellow  stars,  they  should  radiate  more  rapidly  and 
advance  more  rapidly  in  spectral  type  thereby,  if,  as 
is  often  assumed,  advance  in  spectral  type  is  a  mere 
function  of  radiation  and  consequent  condensation. 

I  venture  to  suggest  that  if  the  view  of  Campbell 
as  to  relative  masses  and  types  of  spectra  is  well 
founded,1  its  significance  in  this  respect  may  be  the 
following.  As  we  do  not  know  that  the  two  compon- 
ents of  a  binary  are  of  similar  constitution,  may  it 
not  be  that  the  bluer  component  has  a  smaller  coeffi- 

1  Not  all  astronomers  are  agreed  that  it  is  the  general  rule  for  the 
smaller  component  of  a  binary  to  be  less  advanced  in  spectral  type, 
but  Campbell's  review  of  the  evidence  seems  very  convincing. 

426 


PLATE  XXVI. 


FIG.  1.     NEBULA  H.  V.  24.     (G.  W.  Ritchey.) 


FIG.  2.     RING  NEBULA  IN  LYRA.     (G.  W.  Ritchey.) 

Photographed  with  5-foot  reflector  of  the  Mount  Wilson  Solar  Ob- 
servatory. Exposures:  Fig.  1,  5  hours,  Seed  23  plate.  1910,  March 
6.  Fig.  2,  45m  Seed  process  plate.  1910,  July  1. 


THE  SUN  AMONG  THE  STARS 

dent  of  radiation  than  the  other?  By  this  I  mean  that 
if  the  two  objects  were  of  equal  size  and  temperature, 
the  blue  star  would  emit  less  radiation.  We  have  no 
evidence  whether  this  state  of  affairs  occurs  for  bodies 
of  stellar  temperatures,  but  decided  differences  of 
radiating  power  were  found  by  Paschen  and  others 
at  moderately  high  temperatures  for  various  solid 
substances,  some  of  which  might  even  have  been 
expected  to  be  approximately  "black  bodies."  As- 
suming this  explanation,  the  blue  stars  might  radiate 
slower,  even  though  of  decidedly  higher  temperatures 
and  larger  surfaces  in  proportion  to  their  masses  than 
their  yellower  neighbors. 

As  to  the  assumed  difference  of  constitution  of  the 
two  components,  Campbell  has  suggested  that,  they 
were  originally  one  object,  which  separated  owing  to 
too  rapid  rotation.  In  such  a  case  might  not  the 
smaller  object  usually  carry  with  it  a  preponderance 
of  the  lighter  elements  which  composed  the  original 
star  or  nebula?  We  have  seen  in  Chapter  VI  that 
the  lightest  elements  lie  furthest  out  in  the  sun,  and 
it  seems  reasonable  to  suppose  that  the  same  holds 
in  the  case  of  a  just  separating  binary,  so  that  per- 
haps they  might  tend  to  accumulate  in  the  bulging- 
out  component  of  smaller  mass.  If  this  is  so,  then 
the  presence  of  a  chromospheric  type  rather  than  a 
photospheric  type  of  spectrum  or,  in  other  words,  the 
assuming  of  the  spectrum  of  early  stellar  type,  should 
naturally  be  associated  with  the  lesser  component, 
because  it  has  preponderatingly  the  light  elements, 

427 


THE  SUN 

hydrogen,  helium,  etc.,  rather  than  the  heavy  metals 
whose  lines  throng  the  solar  spectrum.  I  do  not 
mean  by  this  to  say  that  the  lesser  star  has  none  of 
the  heavy  elements,  and  the  greater  star  all  of  them. 
Rather  that  the  lesser  star  has  so  large  a  supply  of  the 
lighter  elements  that  they  effectually  screen  by  their 
scattering  of  light l  the  radiation  of  the  heavier  ones 
lower  down,  just  as  it  is  probable  that  the  elements 
of  the  platinum  group  are  obscured  in  the  sun  as  ex- 
plained in  Chapter  VI.  In  the  greater  star  the  ele- 
ments hydrogen,  helium,  etc.,  while  present,  I  sup- 
pose to  be  less  plentiful,  so  that  the  heavier  metals 
occupy  practically  a  surface  position,  and,  hence, 
give  their  typical  spectra. 

But  it  will  be  urged  that  this  view  implies  too 
much,  and  does  not  take  into  account  the  progressive 
change  of  spectral  type  shown  to  occur  with  increas- 
ing age  of  binaries.  In  other  words,  that  it  would 
imply  that,  once  a  blue  star,  never  a  solar  star. 
Before  answering  this  objection  let  us  examine  Table 
I  of  this  book,  which  shows  that  the  four  outer,  and 
according  to  the  Laplacian  hypothesis  probably  old- 
est, planets  of  the  solar  system  are  all  of  low  density, 
even  lower  (notwithstanding  their  probably  low  tem- 
peratures) than  that  of  the  enormously  hot  sun,  and 
four  times  as  low  as  the  densities  of  the  four  inner 
planets.  Majr  not  some  support  of  the  view  just 
advanced  be  gained  from  this  circumstance?  Were 
not  these  planets  constructed  from  the  solar  nebula, 

1  See  Chapter  VI. 
428 


THE  SUN  AMONG   THE  STARS 

accumulating  with  them  a  preponderance  of  the 
lighter  solar  constituents?  If  so,  did  not  their  forma- 
tion tend  to  advance  the  type  of  the  solar  spectrum? 
May  not  other  stars,  binary  stars  not  excluded, 
whether  primaries  or  secondaries,  also  give  rise  to 
relatively  small  planets  and  satellites,  thereby  losing 
their  lighter  surface  materials,  and  hence  advancing 
themselves  in  spectral  type?  Such  planets,  if  of 
small  masses  relatively  to  their  primaries,  could  not 
be  observed,  and  hence  may,  for  all  we  know,  exist. 

Dr.  Johnstone  Stoney  showed  many  years  ago  that 
the  lighter  elements  gradually  escape  from  atmos- 
pheres according  to  the  kinetic  theory  of  gases,  and 
the  more  rapidly  the  higher  the  prevailing  tempera- 
tures. He  explained  by  this  means  the  absence  of 
water  vapor  from  Mars,  of  all  gases  from  the  moon, 
and  of  helium  and  hydrogen  in  marked  quantity  from 
the  earth.  May  not  this  process,  favored  as  it  must 
be  by  the  high  temperatures  of  the  blue  stars,  aid  in 
course  of  ages  to  divest  them  of  hydrogen,  helium, 
etc.,  and  thereby  tend  to  advance  their  type? 

Without  wholly  accepting  until  we  have  fuller 
evidence  the  relation  pointed  out  by  Campbell  in 
regard  to  spectral  types  and  masses  of  binaries,  the 
suggestion  just  made  as  to  a  possible  path  of  stellar 
evolution  is,  of  course,  not  limited  in  its  application  to 
the  cases  of  binary  stars.  It  may  be  that  all  the  blue 
stars  are  of  early  spectral  type  because  their  elements 
of  high  atomic  weight  are  obscured  by  the  lighter 
gases,  hydrogen,  helium,  etc.,  and  that  with  the  es- 

429 


THE  SUN 


cape  of  these  gases  to  space  or  the  formation  of  satel- 
lites, the  spectral  type  will  be  advanced  to  the  solar 
stage,  and  from  that  by  cooling  to  the  Antarian.1 

Returning  from  these  perhaps  too  presumptuous 
digressions,  I  shall  finally  call  attention  to  some  data 
noted  by  Prof.  Kapteyn  as  perhaps  "  valuable  in  the 
classification  of  the  stars  in  the  order  of  their  evolu- 
tion."2 He  remarks  first  the  progressive  increase  of 
"  peculiar  "  stellar  velocities 3  for  stars  of  the  advan- 
cing spectral  types.  In  the  following  little  table  he 
sums  up  the  results  thus  far  available. 

TABLE  XXXIV. — Spectral  types  and  velocities  in  space 


Type  of  spectrum  or  object 

Peculiar  radial 
velocity  per  second 

Number 

B  to  B  9                             

km 
6.5 

64 

AtoA5  

12.6(11.2) 

18 

Fto  F  

14.5 

17 

G  to  G5 

12.6 

26 

K  toK5 

•  15.4 

55 

Ma      

19.3 

6 

Planetary  nebula  

26.8 

13 

Orion  nebula    

0.1 

1    . 

N              

13.1 

8 

L         

3.7 

2 

1  Consult,  in  this  connection,  T.  J.  J.  See's  "Researches,"  vol.  ii, 
p.  589. 

2  Contributions,  Mount  Wilson  Solar  Observatory,  No.  45. 

3  By  this  is  meant  "the  velocity  freed  from  that  part  which  is  due 
to  the  motion  of  the  solar  system  through  space." 

430 


THE  SUN  AMONG   THE  STARS 

He  then  discusses  the  " peculiar"  proper  motions 
of  over  2,000  stars  and  finds  the  ratio  of  their  average 
magnitudes  to  the  solar  motion  as  follows : 

Ratio        Number 

Type  I  (B,  A) 1.02         1144 

Type  II  (F  to  K) 1.46         1093 

Type  unknown 1 . 45          381 

He  concludes  that  the  ratio 

Average  linear  velocity  of  the  F,  G,  K  stars 

Average  linear  velocity  of  the  A  stars 
cannot  be  smaller  than  1.3. 

It  is  greatly  to  be  regretted  that  there  are  not  more 
radial  velocities  for  nebulae  of  the  Orion  type  known 
as  yet,  but  the  evidence  seems  to  indicate  that  such  a 
nebula  is  to  be  regarded  as  practically  stationary,  and 
that  the  stars  of  advancing  spectral  types  are  affected 
by  progressively  greater  and  greater  motions  in  space, 
but  that  the  planetary  nebulae  are  not  to  be  classed 
with  the  nebulae  of  Orion  type,  but  at  the  other  end 
of  the  chain. 

Kapteyn  says:1  "The  phenomenon  of  the  increase 
of  velocity  with  the  evolutional  stage  of  the  stars 
must  give  rise  to  speculation  as  to  its  cause.  The 
observational  results  contained  in  our  table  naturally 
lead  us  to  conclude  that  the  matter  from  which  the 
stars  originate  must  have  little  or  no  velocity.  How 
is  this  possible  under  the  influence  of  the  combined 
attraction  of  the  rest  of  the  system?  Is  it  not  as  if2 

1  Contributions  of  the  Mount  Wilson  Solar  Observatory  No.  45. 

1 "  We  need  not  necessarily  make  the  hypothesis  that  really  primor- 
dial matter  is  not  subject  to  gravitation.  If,  for  instance,  as  was 
suggested  to  me  by  a  friend,  the  tenuity  of  this  matter  were  such  that 

431 


THE   SUN 

gravitation  had  no  effect  on  the  cosmical  matter  in  its 
primordial  state?  If  this  be  so — as  soon  as  matter 
changes  from  this  state  to  another  in  which  gravity 
begins  to  act,  or  to  act  freely,  motion  will  arise,  and  it 
is  evident  that,  as  a  rule,  the  motion  must  be  acceler- 
ated, at  least  during  immense  periods,  so  that  the 
longer  the  period  elapsed  since  the  birth  of  the  stars 
the  greater  must  be  their  average  velocity." 

After  calling  attention  to  the  argument  from  binary 
systems,  which  we  have  already  considered,  Kapteyn 
mentions  the  two  great  star  streams  which,  according 
to  his  researches,  embrace  the  stars,  and  notes  that, 
as  shown  by  Dyson,  the  stars  of  type  I  "  diverge  less 
from  the  general  drift  of  the  two  streams  than  the 
other  stars. "  Such  a  result  harmonizes  with  the  view 
that  the  Orion  stars  are  relatively  young.  But,  says 
Kapteyn:  "Not  only  this.  Observation  shows  fur- 
ther that  for  the  Orion  stars  the  stream  velocity  is 
small ...  as  compared  with  . . .  the  rest  of  the  stars. 
Apart  from  the  advantages  that  we  may  derive  from 
this  result  for  the  classification  of  the  stars  in  the 
order  of  their  evolution,  it  has,  I  think,  a  great  im- 
portance in  its  bearing  upon  the  question  of  the  gen- 
it  were  very  materially  hindered  in  its  motion  by  the  matter  which  we 
must  assume  as  filling  the  universe  in  order  to  explain  the  phenom- 
enon of  selective  absorption  of  light  recently  found,  the  velocity 
of  this  matter  could  not  exceed  the  value  for  which  the  resistance  is 
equal  to  the  total  attraction.  .  .  .  Other  suppositions  may  probably 
be  made  of  forces  which,  in  the  primordial  state  of  matter,  counter- 
act gravity.  But  it  is  evident  that  in  such  cases  where  gravity  is 
just  counterbalanced  by  another  force,  things  happen  as  if  there  were 
no  force  at  all."  (May  not  light  pressure  be  such  a  force?) 

432 


THE  SUN  AMONG   THE  STARS 

eration  of  the  star  streams  themselves.  For  it  proves 
that  the  streaming  motion,  too,  is  not  an  initial 
motion,  but  one  generated  at  an  epoch  which,  for  the 
stars  of  any  one  type,  must  be  placed  at  a  time  rel- 
atively but  little  preceding  the  time  when  they 
passed  through  the  Orion-type  stage." 

The  results  of  Kapteyn  aid  greatly  to  convince  us 
that  the  progress  of  evolution  is  from  the  Orion  type 
of  nebula  at  the  beginning,  to  the  fourth  type  star  at 
the  end,  and  not  the  opposite,  in  the  lapse  of  time. 
For  we  shall  see  from  them  that  there  is  a  real  prog- 
ress from  one  stage  to  another,  marked  by  the  gradual 
march  of  velocities.  It  only  remains  to  show  that 
the  march  is  in  the  supposed  direction  and  not  its 
opposite,  and  for  this  purpose  the  study  of  one  part 
of  the  course  is  as  good  as  another.  Now  we  know 
that  the  stars  of  the  second  type  resemble  the  sun's 
photosphere,  and  those  of  the  third  type  the  sun 
spots  in  their  spectra,  and  that  this  difference  is 
brought  about  in  the  sun  by  the  mere  reduction  of 
temperature.  A  reduction  of  temperature,  however, 
must  finally  occur  when  a  star  exhausts  its  sources  of 
energy.  Hence,  the  third  type  stars  must  probably 
be  a  later  stage  of  evolution  than  the  second,  and  the 
progress  of  evolution  is  therefore  from  the  Orion 
nebula  to  the  fourth  type  star.  This  conclusion  is 
supported  also  by  Campbell's  discussion  of  binary 
stars. 

Various  considerations,  then,  recommend  the  view 
that  the  stars  are  formed  from  nebulae,  take  first  the 

433 


THE  SUN 

Orion  type,  and  pass  on  with  age  to  the  solar  class, 
and  thence,  with  cooling,  to  the  Antarian  stage  anal- 
ogous with  sun  spots  in  spectrum.  We  may  suppose 
that  a  still  more  advanced,  and  usually  final  stage,  is 
the  cold  one  of  which  the  earth  and  moon  are  types. 
It  is  speculating  far  from  the  sure  ground  of  observa- 
tion to  say  it,  but  do  not  these  conclusions,  and  es- 
pecially Kapteyn's  discussion  of  the  velocities  of 
nebulae  and  stars,  indicate  that  the  entire  stellar 
system  arose  from  a  sort  of  formless,  relatively 
motionless,  chaos,  and  will  at  length  reach  a  dark 
and  unknown  end? 


CONCLUSION 

In  our  study  of  the  sun  and  its  relations  with  the 
earth  and  stars,  the  discoveries  of  two  types  of  inves- 
tigators have  come  prominently  before  us.  To  one 
class  belong  those  geniuses  whose  roving  minds  in- 
cline them  to  try  this  and  that  new  thing,  and  whose 
acute  perceptions  enable  them  to  turn  even  their  most 
random  observations  into  glorious  discoveries.  In- 
vestigation to  them  is  like  happy  exciting  play  to  a 
child.  These  have  their  place  and  their  ever-present 
reward.  To  another  class  belong  the  patient  ob- 
servers and  philosophers  who,  from  a  love  of  science 
and  a  sense  of  duty  to  their  age  and  to  posterity, 
have  gradually  enlarged  by  tedious  observation  and 
laborious  analysis  that  precious  store  of  exact  knowl- 
edge whose  value  time  cannot  impair  but  can  only 
enhance.  The  men  of  both  classes  are  deserving  of 
admiration,  the  former  for  their  brilliancy,  the  latter 
for  their  perseverance.  As  those  of  the  former  class 
are  continually  receiving  their  meed  of  praise  from 
their  contemporaries,  it  will  not  be  amiss  to  offer  our 
tribute  to  the  others,  and  recall  to  mind  the  work  of 
Newton,  whose  immortal  "Principia"  he  suffered  to 
remain  unknown  until  by  the  importunity  and  finan- 
cial means  of  his  friend  Halley  it  came  at  last  to  pub- 
lication; Laplace,  Gauss,  Ilansen,  Newconib,  and 

435 


THfe  SUN 

many  more  who  erected  the  wonderful  edifice  of 
mathematical  astronomy  on  the  foundation  of  New- 
ton's law  of  gravitation;  the  long  series  of  observers 
from  Galileo  down,  whose  sun  spot  records  were  com- 
bined by  Wolf  with  such  rare  skill,  after  the  patient 
work  of  twenty  years  by  Schwabe  had  indicated  the 
sun-spot  cycle;  Carrington,  Spoerer,  and  the  others 
whose  numerous  observations  revealed  the  law  of 
rotation  of  the  sun;  Kirchhoff,  A.  Angstrom,  and  our 
own  wonderful  Rowland,  whose  spectrum  researches 
are  the  foundation  of  solar  physics;  the  unremem- 
bered  army  of  meteorological  observers  whose  plod- 
ding records  are  sometimes  scoffed  at  by  the  more 
brilliant,  but  which  nevertheless  share  in  the  en- 
hancement of  value  produced  by  generous  Time; 
Bradley,  the  father  of  exact  stellar  observation,  whose 
thousands  of  accurate  star  places  are  priceless  to 
modern  astronomy;  Argelander,  whose  enormous 
work,  the  "  Durchmusterung "  of  the  northern  stars 
is  yet  in  the  prime  of  its  usefulness;  Huggins,  whose 
pioneer,  yet  long  sustained,  investigations  in  as- 
tronomical .spectroscopy  laid  the  foundation  of  the 
study  of  stellar  evolution. 

These  men  and  many  more  who  were  actuated  by 
the  same  motives  have  passed  on,  but  their  work  still 
lives.  There  still  remains,  and  ever  will  remain  in 
solar  and  stellar  investigation,  room  for  such  work; 
and  on  the  thorough  doing  of  it  in  our  time  the  won- 
derful flowers  of  future  discovery,  whose  beauty  our 
eyes  cannot  see,  or  our  imaginations  picture,  must 

436 


CONCLUSION 

largely  depend.  If  we  now  had  such  long,  unbroken, 
and  accurate  series  of  meteorological  records  of  nu- 
merous stations  in  all  parts  of  the  world,  on  land  and 
sea  and  in  the  air,  as  posterity  must  depend  on  us  to 
supply;  if  we  now  had  those  long-kept,  numerous, 
and  accurate  observations  of  stellar  parallaxes, 
brightness,  forms  of  spectra,  velocities,  and '  other 
data  which  Kapteyn  longs  for,  but  in  vain;  if  we  now 
had  accurate  measurements  of  the  solar  constant  of 
radiation  going  back  centuries;  in  short,  if  we  could, 
as  we  find  the  need  of  it,  consult  the  records  of  the 
Past  to  verify  the  surmises  of  the  Present,  then  solar 
and  stellar  knowledge  would  advance  with  such  leaps 
and  bounds  that  we  could  soon  see  the  great  pano- 
rama of  the  universal  evolution  unroll  before  us. 

The  child  is  said  to  long  to  grasp  the  moon.  Who, 
in  his  maturer  years,  has  never  wished  that  he  might 
stand  upon  the  moon,  and  watch  the  earth  at  full,  a 
glorious  planet  of  the  night,  four  times  as  far  from 
rim  to  rim,  and  twice  as  bright  in  every  part  as  is  the 
moon  herself!  Who,  thinking  more  gravely,  has  not 
wished  sometimes  he  had  been  born  in  later  years, 
when  he  could  share  the  fuller  understanding  yet  to 
come?  Shall  we  not  live  in  hope  that  if  we  worthily 
contribute  to  that  happy  end,  we,  too,  may  join  with 
that  great  company  whose  patient  and  sound  labors 
have  given  us  what  we  know,  and  in  a  future  life  with 
them  may  see  unrolled  the  wider  view  which  here  we 
long  to  see  in  vain? 


INDEX 


Abbot,  C.  G.,  108,  190. 
pyrheliometer,    70. 
solar  theory,   23G-279. 
Mrs.  C.   G.,   132. 
Aberration  of  light,  22. 
Absorption,  and  radiation,  C4. 

atmospheric,  291,  308. 
Adams,  W.,   379. 

W.  S.,  25,  100-104,  124-127, 
180-182,     205-209,     213, 
255-258,  268.      , 
Air  mass,  280-290. 
Anderson,  233. 
Angstrom,  A,  436. 

K.,  pyrheliometer,  77. 
Apex  of  solar  motion,  395. 
Arago,  142. 
Archimedes,  363. 
Arctowski,   190,  317. 
Argelander,  436. 
Armstrong   electrical     machine, 

213. 
Assimilation     of      carbon      by 

plants,  335-344. 
Atmosphere,  earth's,  extinction, 

74,  294-297. 
height,  74. 
spectrum,  88. 

Atomic   weights    and   spectrum 
intensities,    91,    93,    169, 
253,  317. 
Aurora  and  sun  spots,  189,  220. 


Ball,  R.,  222. 

Balmer  series,  170,   173. 

Becker,  G.   F.,  273,  278. 

L.,   132,  133. 
Belopolsky,  135. 
Bessel,  392. 

Bigelow,  111,  135,  190,  317. 
Body,  "black,"  64,  65. 
Bolograph,  83,  292. 
Bolometer,   81,    84,    86. 
Boss,  396. 

Bouguer  formula,  74,  289,  293. 
Boussingault,    358. 
Boyle,  Willsie  and,  376-379. 
Boys,   C.  V.,  28. 
Bradley,  396,  436. 
Brown,  358. 

and  Escombe,  339. 

E.    W.,    on    Moon's    motion, 

7,  16. 

Buffon,    363. 

Buisson,  96,  99,  104,  256. 
Bunsen,  38,  39,  87,  98,  349. 
Burning  mirrors,  363. 

Calcium,    circulation     in     sun, 
101. 

hydride,  209. 

level,  97,  119,  126,  127. 

lines,  wave  length,  102 

spectroheliograms,  118. 
Calorie,  68. 


30 


439 


INDEX 


Calvert,  132. 

Campbell,    135,    169,    180,    263, 

395-398,  406,  422-429. 
Carbon,  in  sun,  93. 

dioxide,  absorption  and  radi- 
ation, 281,  291. 
and  plants,    335-344. 
Carrington,    32,    122-126,    153, 

192,  198,  436. 
Cavendish,  28. 
Cebrian,  Molera  and,  375. 
Chamberlin,  274,  276,  330,  413- 

418. 

Chase,  393. 
Chemistry,  of  stars,  405. 

of  sun,  87,  91. 
Chevalier,    124,    126. 
Christie,  195. 
Chromosphere,    flash    spectrum, 

168,  177-179. 
heights,  171,  172. 
in  daylight,  137,  142,  149. 
Spectrum,   88,    143,  145,   180, 

233. 

in  daylight,  180,  181. 
Clark,  381. 

Coast  survey,  U.  S.,  18. 
Coelostat,   36. 

Comparator  measurements,   61. 
Convection  currents  in  sun,  100, 

267. 

versus  radiation,  103. 
Cook,  Captain,  14. 
Copernicus,  1,  392. 
Cornu,  21. 
Corona,    solar,    131-136,    263- 

265. 

Coronium,  134. 
Cortie,  210. 
Cranks,  8. 
Cyanogen  in  the  sun,  94. 


Darwin,  G.,  413,  424. 

De  Candolle,  355. 

De   Saussure,   364,  374. 

Deslandres,  117,   135,  167. 

Diffraction,  51,  52. 

Dispersion,  anomalous  and  reg- 
ular, 233. 

Dixon,  Mason  and,   14. 

Doppler  effect,  23,  41,  42,  88, 
100,  124,  257,  397,  401, 
422,  430. 

Dorsey,  21. 

Double  reversal,  147,   148. 

Duner,  124,  126. 

Dyson,  135,  169-171,  432. 

Earth, \   dimensions    and    mass, 

18,  27. 
temperature     and     radiation, 

308-316. 

and  sun's  variability,   317. 
Ebermeyer,   340. 
Eclipse,  solar,   128-131. 
of  1868,  137. 
of  1900,  132. 
of  1905,  132,  173-179. 
of  1908,  133,   135. 
Efficiency,    thermodynamic,  387. 
Elements,       chemical,       boiling 

points,  238. 
in  sun,  87,  89-91,  252. 
represented    in     flash-spec- 
trum,     169,      170,     177, 
179. 

Elkin,  12. 

Ellerman,  118,   120,   167. 
Emission,  selective,  71. 
Encke,  15. 

Eneas,  solar  machine,  367-372. 
Energy,  available  quantity,  so- 
lar, 383-387. 


440 


INDEX 


Energy,  spectrum,  solar,  68,  82, 

292. 

over  sun's  disk,  107,  109. 
Engelmann,  342. 
Enhanced   lines,   100,    104,   105, 

207. 

Ericsson,  366. 
Eros,  12,   16,  17. 
Escombe,  Brown  and,  339. 
Etiolation,  345. 
Evershed,  99,  115,  166-109,  177, 

209,   210,   213,  244,  255. 
Exner,  300-304. 
Eye,     as     optical     instrument, 

84. 
Eye-pieces,  solar,  33,  34. 

Fabricius,  183. 
Fabry,  96,  99,   104,  256. 
Faculae,    position   and    appear- 
ance, 85,  202. 
Faye,  123. 
Fenyi,   152. 
Fizeau,  19. 
Flacculi,   118,  121. 

motion,  122. 
Flash-spectrum,  168,  170. 

of   1905,    173-180. 
Fluorescent  spectrum,  264. 
Fluorite,  transparency,  56. 
Foucault,  20. 
Fowle,  108,  190,  317. 
Fowler,  209. 
Fox,    100,    122,    124,    126,    198, 

270. 
Fraunhofer,  39. 

lines,  82,  87,  98. 

principal  solar,  90. 
Fresnel  rhomb,  44,  211.. 
Frost,   169,  172,  204,  205,  398, 
407. 


Gale,  208,  -209. 

Galileo,  1,  183,  436. 

Gas,  levels  in  stars,  426-429. 

spectrum,  71,  249. 
Gauss,  435. 
Gegenschein,  400. 
Geography,  plant,  346-354. 
George  III.,  13. 
Gill,  11. 

Glacial  periods,  273-277,  322. 
Glass,  transparency,  56,  375. 
Gould,  317. 
Grant,  142. 

Granulation,  solar,   202. 
Grating,  action,  51. 

concave,  54. 

plane,   55. 

ruling,  52. 
Gravitation,  2. 

constant  of,  28,  29. 

is  universal,  400. 

Hale,  59,  61,  117,  118,  124-126, 
181,    182,  208,  209,  211, 
212,  268. 
Halm,   24,    124,    126,    185,  256, 

271,  397. 

solar  theory,  225-228. 
Hamij  312. 
Hansen,  15,  435. 
Hartmann,  234. 

Harvard,  classification  of  stel- 
lar spectra,  409. 
college  observatory,  23. 
Hastings,  221,  226. 
Heat  and  radiation,  64. 
Heliomicrometer,  60. 
Heliotropism,  354. 
Helium,   in  chromosphere,    170, 
prominences,   139. 
stars,  409. 


441 


INDEX 


Helmholtz,  226,  248,  277,  278, 

413. 

Hercules,  constellation,  xx,  296. 
Herschel,  137,-  141. 

J.,  33,  364. 

W.,  396. 
Hinks,  13. 
Hipparchus,  5. 
Hoesen,  363. 
Holden,  135. 

Hot-box,  364,  376-379,  389. 
Hough,  397. 

Huggins,  141,   142,  425,  436. 
Humphreys,  43,  99. 
Hydrogen,  in  stars,  409. 

in  sun-spots,  206. 

level  in  sun,  97,  119,  127. 

spectroheliograms,    1*18. 

Insolation,  march  of,  285. 
Intensities,    spectrum,    and    at- 
omic weights,  89,  91,  93. 
Interferometer,  96. 
Iron,  level  in  sun,  97. 

Janssen,  137,  138,  142,  168,  203. 
Jewell,  43,   99,    115,   169,    171- 

173,  255. 
Josse,  278. 

Julius,  xxiii,  182,  259,  260,  262. 
Jupiter  and  sun-spots,  188. 
satellites  and  sun's  distance, 

23. 

Kapteyn,    xxiv,    396,    430-433, 

437. 

Kayser,   93,  254. 
Keeler,  232. 
Kepler,  4-6,  27,  402. 
Ketchum,  376-378. 
King,  208. 


Kirchoff,  xxiii,  38,  39,  64,   87, 

98,  436. 

law,  65,  71,  208. 
Kniep,  and  Minder,  344. 
Knopf,  232. 
Koppen,  190,  317. 
Krigar-Menzel,  29. 
Kustner,  24. 

Lambert,  formula,  74. 
Lampblack  absorption,    386. 
Lane,  law,  224,  255. 
Langley,  81,  130,  200-206,  226, 

295,  311,  373. 
Laplace,  6,  412,  413,  435. 
Layer,  reversing,  88,  170,  178. 
Levels   of    spectrum    formation, 

97,     102,     118-120,     126, 

127,  171. 
Leverrier,  16. 
Lewis,  135,  136,  263. 
Light,  and  plant  growth,  342- 

346. 
direction   and    plant  growth, 

354-357. 

velocity,  19-21,  46. 
year,  294. 

Limb  spectrum,  105. 
Littrow  spectroscope,  58. 
Lockyer,     90,     137,     139,     142, 

147,    168-171,    185,    195, 

196,  222,  224,  255. 
Lord,  169. 

Magnesium  hydride,  209. 
Magnetism,  effect  on  spectrum, 

43. 

in  sun-spots,  211-213. 
terrestrial      and      sun-spots, 

187-191. 
Manson,  273. 


442 


INDEX 


Mars,  and  solar  parallax,   11. 
Mascari,    195. 
Maskelyne,  ,14,  27. 
Mason  and  Dixon,  14. 
Maunder,   126,   189,   198. 
Measurement,  spectrum,  61. 
Michell,  28. 
Michelson,  A.  A.,  21,  96. 

W.  A.,  79,  260. 
Minder,  Kniep  and,  344. 
Mira  Ceti,  variability,  404. 
Mirrors,  burning,  363. 

reflecting  power,  388. 
Mitchell,    S.  A.,    168-172,    180, 
181,  255. 

flash-spectrum  of   1905,    173- 

180. 

Mohler,   43,  99. 
Moissan,  237,  239. 
Molera  and  Cebrian,  375. 
Moore,  133. 
Motion  in  line  of  sight,  40,  127, 

213,  397,  422. 
Mouchot,  366. 

Moulton,  271,  330,  413-418. 
Mount  Whitney,  69,   296-298. 
Mount  Wilson,  xxiii,  57,  69,  89, 
118,   121,  210,  296-298. 

Solar  Observatory,  57,  60. 

Natanson,  xxiii,  305,  343. 
Naval  Observatory,  U.  S.,   132, 

173. 

Nebulae,  and  stars,  418-421. 
solar,  276,  326,  412-418. 
velocities,  430. 
Neptune,  distance,  xviii,  3. 

sun's  attraction,  2,  3. 
Newcomb,  8,  15,  16,  21,  27,  185, 
190,   193,  236,  278,   317, 
396,  400,  420,  435. 


Newton,  xviii,  6,  28,  86,  435. 

Nicol  prism,  44,  211. 

Nitrogen    required    by    plants, 

334. 
Nordmann,  190,  317. 

Olmsted,  209. 

Oltmans,  356. 

Ore  reduction,  381. 

Orion  stars,  408,  419,  430-432. 

Oxygen  in  sun,  92. 

Parallax,  solar,  9,  12-16,  22-25. 

stellar,  392. 
Paschen,  427. 
Permian  glaciation,  273-277, 

322-329. 
Perot,  96. 

Perrine,  135,  169,  263. 
Perrotin,  21. 
Perry,   130,  152,   198. 
Perseus,  new  star  in,  xix.    • 
Pfeffer,  358. 
Photography  of  sun,  34,  86. 

spectrum,    57. 

Photosphere,       spectrum,       ele- 
ments, 91. 
extent,  86. 
sun's,  85,  217,  220. 
Pickering,  E.  C.,  226,  228,  403, 

409. 

Pifre,  366. 

Planck,  law,  66,  251,  426. 
Planetesimal  hypothesis,  414. 
Planets,  minor,  12,  13,  17. 

principal  data,  3. 
Plants  and  light  direction,  354- 

357. 

and  the  sun,  331-361. 
as  energy  accumulators,  357- 
361.     ' 


443 


INDEX 


Plants,    chemical   requirements, 
331-335. 

geography,   346-354. 

light  requirements,  349-354. 

rest  periods,   348. 
Plateau,   31. 

Platinum  black  absorption,  387. 
Pleiades,  398. 
Pogson,  137. 
Poincare,  423. 
Polarization  of  corona,  134. 

of  light,  44. 
Potassium  in  sun,  92. 
Pouillet,  73,  74. 
Pressure,    effect    on    spectrum, 
43,  104. 

in  sun,  98,  213. 
Pringoheim,  261-264. 
Prism   action,  47-49. 
Procyon,  40. 

Prominences  and  sun-spots,  149, 
.    153,  155. 

classification,   156. 

detached,  161. 

eruptive,  157,  162,  163. 
rapid  change,  164,  165. 
spectrum,  162. 

in     full     daylight,     138-142, 
149. 

magnitude,  155. 

number  and  distribution,  152, 
153. 

quiescent,   159-161. 

solar,  121,  213. 

spectroheliograms,  166. 

spectrum,    138-143. 
Pyrheliometer,  errors,  78. 

Michelson's,  W.  A.,  79. 

Pouillet's,  73. 

silver  disk,  75,  76. 

waterflow,  79. 


Quartz,  transparency,  56. 

Radiation    and     plant     growth, 
342-346. 

and  temperature,  41,  64,  70, 
308-316. 

measurement,  283,  284. 

nature,  63,  286. 

over  sun's  disk,  107,   110. 

solar  constant  of,  xx,  75,  295, 
298. 

unit,  68. 

versus  convection  in  sun,  103. 
Radiator,  perfect,  64-66. 
Radium   in    sun,    94,    272. 
Rayet,  137,  138. 
Rayleigh,  xxiii,  241,  304. 
Reed,  148. 

Reflecting  powers,  388. 
Refraction,  anomalous  and  reg- 
ular, 233. 

circular,  229. 

indices,  46. 

law,  48. 
Reinke,  342. 
Respighi,  155. 
Reversal,  double,  147,  148. 
Reversing  layer,  88. 

pressures,  99. 

thickness,   171. 
Ricco,  166,  195. 
Rice-grains,  85. 
Richarz,  29. 
Ritchey,  418. 
Roberts,  403. 
Roche,  413. 

Rock-salt,  transparency,  56. 
Romer,  23. 
Rosa,  21. 
Roscoe,  302,  349. 
Rosse,  311. 


444 


INDEX 


Rotation,  solar,  122-127. 
Rowland,   52,   62,   89,   90,    169, 

175,  212,  254,  268,  436. 
"  Preliminary  Table,"  89. 
wave  lengths,  corrections,  95, 

96. 

Runge,  93,   176,  254. 
Russell,  404. 

St.    John,    89,    101,    121,    127, 
213,260. 

Salisbury,  274. 

Salt,  rock-,  dispersion,  233. 
transparency,  56. 

Samson,  23,   271. 

Saros,  129. 

Scattering  of  light  by  gases,  241. 

Schaberle,  265. 

Schehallien,  28. 

Scheiner,     discovery     of     sun- 
spots,  183. 
J.,  113,  247,  410,  421,  426. 

Schmidt,  xxiii,  244. 
solar  theory,  228. 

Schramm,  303. 

Schultz,  237. 

Schuster,   xxiii,    188,   236,   242, 
271,  301. 

Schwabe,  184,  436. 

Schwartzchild,    103,    108,    203, 
204,   237,   247. 

Sehwendener,  338. 

Secchi,  142,  152,  156,  161,   166, 
196,    198,  236,  407-409. 

See,  T.  J.  J.,  103,  226,  247,  417, 
430. 

Shackelton,   168,   1G9. 

Shearman,  190. 

Shuman,  376. 

Sidgreaves,   152,   198. 

Silver,  reflecting  power,  56. 


Sky,  light  of,  299-307,  349-354. 
Slocum,  166. 

Smithsonian    Institution  obser- 
vations, 69,  86,  203,  284, 
298,  321,  384,  387. 
Smithsonian    Observatory,    Mt. 
Wilson,  37. 

pyrheliometer,  75. 
Smoke,  absorption,  386. 
Snow  telescope,  60. 
Sodium,   anomalous   dispersion, 

233. 

Spectroheliograms,    118-120. 
Spectroheliograph,  59,  117. 

and  prominences,  166. 
Spectroscope,  45. 

a  vitesse,  117. 

grating,  53,  54. 

prismatic,  50. 
Spectrum,  analysis,  39. 

appearance,  38. 

corona,  134. 

limb,  105. 

lines,     broadening     in     sun- 
spots,  211. 
telluric,  43. 

meaning,  39-41. 

measurement,  61. 

negative  elements,  95. 

prominence,    138,    145-147. 

solar,  extent,  86. 

sun-spot,  203-210. 
Spoerer,    122,     123,     126,     193, 

194,  220,  436. 
Stark,  261. 

Stars    and    nebulae,    399,    418- 
421. 

density,  403. 

distance,  xix,  392. 

double,  400-404. 

evolution,  418-434. 


445 


INDEX 


Stars,    fission    of,  429-433. 
groups,  398. 
magnitudes,  394. 
mass,  402,  424. 
Orion,  408,  419. 
solar,  408,  419. 
solar  and  Antarian,  433. 
spectra,  405-412. 

and  velocities,  430. 

classification,  407-409,  422. 

energy,  distribution,  410. 
spectroscopic,     binary,      401, 

422-429. 
streams,  432. 
I       Stefan^W.  67,  110,  250. 


rnerf,  7. 
Stomata,  338. 
Stone,  317. 
Stoney,  222,  420. 
Stralonoff,  123,  126. 
Struve,  392. 

Sun,  among  the  stars,  391-434. 
and  plants,  331-361. 
axis,  122. 
brightness,  distribution,  105- 

107. 

compared  to  Mira  Ceti,  404. 
cooking,  379. 
corona,  263. 
density,   30. 

dimensions  and  mass,  26,  27. 
distance,    geometrical    meth- 
ods, 9,  12,   13,   15. 
gravitational  methods,  15. 
summary,  25,  26. 
velocity   of   light   methods, 

22,  23. 
energy  supply,  271,  277,  383- 

387. 

gaseous,  xxiii,  30,  217,   229, 
237. 


Sun,  general  features,  xviii. 
heaters   and   reservoirs,   375- 

380. 

machines,  366-372,  376-379. 
motion  among  stars,  395. 

in  space,  xx. 
nature  of,  Abbot,  236-279. 

Hahn,  225-228. 

Julius,  233-236. 

Schmidt,  228-233. 

Young,  215-225. 
ore  reduction,  381. 
origin,  412-418. 
photosphere,  edge  darkening, 
249. 

levels,  250. 

nature,  246. 

"  rice-grains,"   247. 

spectrum,  249. 

temperature  variation,  250. 

thickness,  244. 
prominence   spectrum,  259. 

velocities,  259. 
radiation,  constant,  75. 

dependence    on     air     mass, 
286. 

measurement,  283,  284. 
rotation,    122-127. 
sharp  boundary,  241. 
spectrum    and    chemical    ele- 
ments, 252. 

bright  line,  259. 

center  and  edge,  255. 

dark  line,  251. 
stellar  magnitude,  394. 
temperature,      70,      109-116, 

265, 
utilization    of    energy,    362- 

390. 

variability,  317. 
Sunflower,  economy,  361. 


446 


INDEX 


Sun-spots    and   associated    phe- 
nomena,  189,   190. 

and    ttocculi,    121,   122. 

and     magnetism,      187,     191, 
211. 

and    prominences,     149,    153, 
155. 

and  solar  rotation,  122. 

and    terrestrial    temperature, 
190,   191,  317. 

coolness   of,   207-210. 

darkness   of,   203,  204. 

discovery,    183. 

distribution  on  sun,  193-19G. 

drift,   192. 

formation    and    life    history, 
196-199. 

length  of  period,  185. 

level,  199. 

motion  within,  213. 

nature  of,  2C7. 

periodicity,  184-188,  192,225. 

pressure    within,    213. 

relative  numbers,  184. 

size,  183. 

spectrum,    203-210. 

titanium  oxide  in,  94. 
System,  solar,  dimensions,  1,  3. 
evolution,  412-418. 
relative  distances,   5. 

Tacchini,  152,  158,  166. 

Tatnall,  254. 

Telescope    for    solar    work,   31, 

58. 

snow,  60. 
tower,  58. 
Temperature  and  plant  growth, 

341. 

and  radiation,  41,  64,  70,  308- 
316. 


Temperature,  attained  in  "  hot- 
box,"    305,  374. 
earth's,      at     high      altitude, 

280-283. 

geologic,  273-277,  322. 
importance  in  sun,  205. 
of  stars,  411. 

progressive,  429,  433. 
of  sun,  109-116. 
terrestrial,  311-316. 
Tennant,  137,  141. 
Thermodynamic   efficiency,   387. 
Thomson,  213. 
Tisserand,  123. 
Titanium     oxide     in     sun,     94, 

209. 
Transmission,  atmospheric,  294- 

297. 
Transparency  of  optical  media, 

56. 

Trouvelot,  156. 
Turner,  25,   132,  133. 
Tyndall,    pyrheliometer,   75. 

Uranium  in  sun,  94. 

Veeder,  220. 

Venus,  transits,    13,    15. 

Very,  311. 

Villager,  108,  203,  204. 

Vogel,  228,  409-412. 

Von  Gothard,   152. 

Vortices,  solar,  120. 

• 
Water,  required  by  plants,  332. 

vapor,  radiation  and  absorp- 
tion, 281,  291. 
Wave  lengths,  accuracy,  62. 

and  levels,  98. 

and  plant  growth,  342. 

and  pressures,  99. 


447 


INDEX 


Wave   lengths,    principal   solar, 

90. 

range,  41,  283. 
Rowland's  89. 

corrected,  62,  96. 
Wheatstone,  bridge,  81. 
Wiedemann,  E.,  255. 
Wien,  displacement  law,  67,  70, 

109,  119,  250. 

Wien-Planck  law,  66,  111,  251. 
Wiesner,  302,  349-354. 
Wilczynski,   271. 
Willsie  and  Boyle,  376-379. 
Wilsing,  113,  123,  271,  410,421, 

426. 
Wilson,  A.,   199,  200. 

W.   E.,  204,  205. 
Witt,  Eros  parallax,  16. 


Wolf,  184,   185,  436. 

sun-spot   numbers,    184,    18 

194,   195. 
Wolfer,  185. 
Wood,  R.  W.,  233,  264. 

Yerkes    Observatory,    132,    16 

407. 

Young,    5,  10,    134,    137,    14' 

100,  161,    165,    168,   19; 

196,  260. 

views   of  sun's   nature,   215 

225. 

Zeeman,  43,  211. 
Zodaical   light,   400. 
Zollner,  142. 


(1) 


THE   END 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


DEC  17  1947 

28Mar'49»f 

ijjun49D| 

•4Dec'58/»H 
REC'D  1_D 

DEC  - 1 SS58 


IRVINE 

INTERUBRARY 


NOV  0  9  1991 

/WO  DISC  NOV  05 '91 

KAY  03 1992 

r ' ' ' .  •  •  ;  '    -  ••     •  • ./ 

APR  0  3  1982 


LD  21-100m-9,'47(A5702sl6)476 


U.C.  BERKELEY  LIBRARIES 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


